diff --git a/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/AudioStartEvent.java b/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/AudioStartEvent.java
index e379f1ce28..6171536771 100644
--- a/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/AudioStartEvent.java
+++ b/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/AudioStartEvent.java
@@ -12,10 +12,13 @@
  */
 package org.openhab.core.voice;
 
+import org.eclipse.jdt.annotation.NonNullByDefault;
+
 /**
  * A {@link STTEvent} fired when the {@link STTService} starts hearing audio.
  *
  * @author Kelly Davis - Initial contribution
  */
+@NonNullByDefault
 public class AudioStartEvent implements STTEvent {
 }
diff --git a/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/AudioStopEvent.java b/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/AudioStopEvent.java
index b053d7a985..835b173f8c 100644
--- a/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/AudioStopEvent.java
+++ b/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/AudioStopEvent.java
@@ -12,10 +12,13 @@
  */
 package org.openhab.core.voice;
 
+import org.eclipse.jdt.annotation.NonNullByDefault;
+
 /**
  * A {@link STTEvent} fired when the {@link STTService} stops hearing audio.
  *
  * @author Kelly Davis - Initial contribution
  */
+@NonNullByDefault
 public class AudioStopEvent implements STTEvent {
 }
diff --git a/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/KSErrorEvent.java b/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/KSErrorEvent.java
index 4f4b65ebff..6448843d28 100644
--- a/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/KSErrorEvent.java
+++ b/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/KSErrorEvent.java
@@ -12,11 +12,14 @@
  */
 package org.openhab.core.voice;
 
+import org.eclipse.jdt.annotation.NonNullByDefault;
+
 /**
  * A {@link KSEvent} fired when the {@link KSService} encounters an error.
  *
  * @author Kelly Davis - Initial contribution
  */
+@NonNullByDefault
 public class KSErrorEvent implements KSEvent {
     /**
      * The message describing the error
diff --git a/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/KSEvent.java b/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/KSEvent.java
index 937313a3fa..2757ccef36 100644
--- a/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/KSEvent.java
+++ b/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/KSEvent.java
@@ -12,10 +12,13 @@
  */
 package org.openhab.core.voice;
 
+import org.eclipse.jdt.annotation.NonNullByDefault;
+
 /**
  * A tagging interface for keyword spotting events.
  *
  * @author Kelly Davis - Initial contribution
  */
+@NonNullByDefault
 public interface KSEvent {
 }
diff --git a/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/KSException.java b/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/KSException.java
index ed484ffff6..6289297699 100644
--- a/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/KSException.java
+++ b/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/KSException.java
@@ -12,11 +12,14 @@
  */
 package org.openhab.core.voice;
 
+import org.eclipse.jdt.annotation.NonNullByDefault;
+
 /**
  * General purpose keyword spotting exception
  *
  * @author Kelly Davis - Initial contribution
  */
+@NonNullByDefault
 public class KSException extends Exception {
 
     private static final long serialVersionUID = 1L;
diff --git a/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/KSListener.java b/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/KSListener.java
index 239f70b5ec..ba5534fe36 100644
--- a/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/KSListener.java
+++ b/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/KSListener.java
@@ -12,6 +12,8 @@
  */
 package org.openhab.core.voice;
 
+import org.eclipse.jdt.annotation.NonNullByDefault;
+
 /**
  * The listener interface for receiving {@link KSEvent} events.
  *
@@ -22,6 +24,7 @@ package org.openhab.core.voice;
  *
  * @author Kelly Davis - Initial contribution
  */
+@NonNullByDefault
 public interface KSListener {
     /**
      * Invoked when a {@link KSEvent} event occurs during keyword spotting.
diff --git a/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/KSService.java b/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/KSService.java
index 3fb2cb3831..697c9a3103 100644
--- a/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/KSService.java
+++ b/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/KSService.java
@@ -15,6 +15,8 @@ package org.openhab.core.voice;
 import java.util.Locale;
 import java.util.Set;
 
+import org.eclipse.jdt.annotation.NonNullByDefault;
+import org.eclipse.jdt.annotation.Nullable;
 import org.openhab.core.audio.AudioFormat;
 import org.openhab.core.audio.AudioStream;
 
@@ -24,6 +26,7 @@ import org.openhab.core.audio.AudioStream;
  * @author Kelly Davis - Initial contribution
  * @author Kai Kreuzer - Refactored to use AudioStream
  */
+@NonNullByDefault
 public interface KSService {
 
     /**
@@ -39,7 +42,7 @@ public interface KSService {
      * @param locale the locale to provide the label for
      * @return a localized string to be used in UIs
      */
-    public String getLabel(Locale locale);
+    public String getLabel(@Nullable Locale locale);
 
     /**
      * Obtain the Locales available from this KSService
diff --git a/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/KSServiceHandle.java b/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/KSServiceHandle.java
index 0513e4e025..d641e3ff63 100644
--- a/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/KSServiceHandle.java
+++ b/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/KSServiceHandle.java
@@ -12,11 +12,14 @@
  */
 package org.openhab.core.voice;
 
+import org.eclipse.jdt.annotation.NonNullByDefault;
+
 /**
  * An handle to a {@link KSService}
  *
  * @author Kelly Davis - Initial contribution
  */
+@NonNullByDefault
 public interface KSServiceHandle {
     /**
      * Aborts keyword spotting in the associated {@link KSService}
diff --git a/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/KSpottedEvent.java b/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/KSpottedEvent.java
index ed939a3013..94d6497770 100644
--- a/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/KSpottedEvent.java
+++ b/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/KSpottedEvent.java
@@ -12,11 +12,14 @@
  */
 package org.openhab.core.voice;
 
+import org.eclipse.jdt.annotation.NonNullByDefault;
+
 /**
  * A {@link KSEvent} fired when the {@link KSService} spots a keyword.
  *
  * @author Kelly Davis - Initial contribution
  * @author Yannick Schaus - Removed AudioSource information
  */
+@NonNullByDefault
 public class KSpottedEvent implements KSEvent {
 }
diff --git a/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/RecognitionStartEvent.java b/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/RecognitionStartEvent.java
index 05d5563251..6180893107 100644
--- a/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/RecognitionStartEvent.java
+++ b/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/RecognitionStartEvent.java
@@ -12,10 +12,13 @@
  */
 package org.openhab.core.voice;
 
+import org.eclipse.jdt.annotation.NonNullByDefault;
+
 /**
  * A {@link STTEvent} fired when the {@link STTService} starts recognition.
  *
  * @author Kelly Davis - Initial contribution
  */
+@NonNullByDefault
 public class RecognitionStartEvent implements STTEvent {
 }
diff --git a/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/RecognitionStopEvent.java b/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/RecognitionStopEvent.java
index 6f75fcfcb3..2fb2ecac66 100644
--- a/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/RecognitionStopEvent.java
+++ b/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/RecognitionStopEvent.java
@@ -12,10 +12,13 @@
  */
 package org.openhab.core.voice;
 
+import org.eclipse.jdt.annotation.NonNullByDefault;
+
 /**
  * A {@link STTEvent} fired when the {@link STTService} stops recognition.
  *
  * @author Kelly Davis - Initial contribution
  */
+@NonNullByDefault
 public class RecognitionStopEvent implements STTEvent {
 }
diff --git a/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/STTEvent.java b/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/STTEvent.java
index 783f4d2868..7c444dfcf5 100644
--- a/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/STTEvent.java
+++ b/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/STTEvent.java
@@ -12,10 +12,13 @@
  */
 package org.openhab.core.voice;
 
+import org.eclipse.jdt.annotation.NonNullByDefault;
+
 /**
  * A tagging interface for speech-to-text events.
  *
  * @author Kelly Davis - Initial contribution
  */
+@NonNullByDefault
 public interface STTEvent {
 }
diff --git a/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/STTException.java b/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/STTException.java
index d0e7510ece..a5b8a26fec 100644
--- a/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/STTException.java
+++ b/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/STTException.java
@@ -12,11 +12,14 @@
  */
 package org.openhab.core.voice;
 
+import org.eclipse.jdt.annotation.NonNullByDefault;
+
 /**
  * General purpose STT exception
  *
  * @author Kelly Davis - Initial contribution
  */
+@NonNullByDefault
 public class STTException extends Exception {
 
     private static final long serialVersionUID = 1L;
diff --git a/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/STTListener.java b/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/STTListener.java
index 9df1076afc..217024fca9 100644
--- a/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/STTListener.java
+++ b/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/STTListener.java
@@ -12,6 +12,8 @@
  */
 package org.openhab.core.voice;
 
+import org.eclipse.jdt.annotation.NonNullByDefault;
+
 /**
  * The listener interface for receiving {@link STTEvent} events.
  *
@@ -22,6 +24,7 @@ package org.openhab.core.voice;
  *
  * @author Kelly Davis - Initial contribution
  */
+@NonNullByDefault
 public interface STTListener {
 
     /**
diff --git a/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/STTService.java b/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/STTService.java
index e7ff736614..e4346d9d9e 100644
--- a/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/STTService.java
+++ b/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/STTService.java
@@ -15,6 +15,8 @@ package org.openhab.core.voice;
 import java.util.Locale;
 import java.util.Set;
 
+import org.eclipse.jdt.annotation.NonNullByDefault;
+import org.eclipse.jdt.annotation.Nullable;
 import org.openhab.core.audio.AudioFormat;
 import org.openhab.core.audio.AudioStream;
 
@@ -23,6 +25,7 @@ import org.openhab.core.audio.AudioStream;
  *
  * @author Kelly Davis - Initial contribution
  */
+@NonNullByDefault
 public interface STTService {
 
     /**
@@ -38,7 +41,7 @@ public interface STTService {
      * @param locale the locale to provide the label for
      * @return a localized string to be used in UIs
      */
-    public String getLabel(Locale locale);
+    public String getLabel(@Nullable Locale locale);
 
     /**
      * Obtain the Locales available from this STTService
diff --git a/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/STTServiceHandle.java b/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/STTServiceHandle.java
index 3c5f39481d..e8b990b672 100644
--- a/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/STTServiceHandle.java
+++ b/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/STTServiceHandle.java
@@ -12,11 +12,14 @@
  */
 package org.openhab.core.voice;
 
+import org.eclipse.jdt.annotation.NonNullByDefault;
+
 /**
  * An handle to a {@link STTService}
  *
  * @author Kelly Davis - Initial contribution
  */
+@NonNullByDefault
 public interface STTServiceHandle {
     /**
      * Aborts recognition in the associated {@link STTService}
diff --git a/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/SpeechRecognitionErrorEvent.java b/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/SpeechRecognitionErrorEvent.java
index d728613d6f..5ab6af1440 100644
--- a/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/SpeechRecognitionErrorEvent.java
+++ b/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/SpeechRecognitionErrorEvent.java
@@ -12,11 +12,14 @@
  */
 package org.openhab.core.voice;
 
+import org.eclipse.jdt.annotation.NonNullByDefault;
+
 /**
  * A {@link STTEvent} fired when the {@link STTService} encounters an error.
  *
  * @author Kelly Davis - Initial contribution
  */
+@NonNullByDefault
 public class SpeechRecognitionErrorEvent implements STTEvent {
     /**
      * The message describing the error
diff --git a/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/SpeechRecognitionEvent.java b/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/SpeechRecognitionEvent.java
index 7af576b771..85a3db6c10 100644
--- a/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/SpeechRecognitionEvent.java
+++ b/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/SpeechRecognitionEvent.java
@@ -12,11 +12,14 @@
  */
 package org.openhab.core.voice;
 
+import org.eclipse.jdt.annotation.NonNullByDefault;
+
 /**
  * A {@link STTEvent} fired when the {@link STTService} recognizes speech.
  *
  * @author Kelly Davis - Initial contribution
  */
+@NonNullByDefault
 public class SpeechRecognitionEvent implements STTEvent {
     /**
      * Confidence of recognized speech
diff --git a/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/SpeechStartEvent.java b/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/SpeechStartEvent.java
index 1dc6703e9e..988c1ee12e 100644
--- a/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/SpeechStartEvent.java
+++ b/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/SpeechStartEvent.java
@@ -12,10 +12,13 @@
  */
 package org.openhab.core.voice;
 
+import org.eclipse.jdt.annotation.NonNullByDefault;
+
 /**
  * A {@link STTEvent} fired when the {@link STTService} starts hearing speech.
  *
  * @author Kelly Davis - Initial contribution
  */
+@NonNullByDefault
 public class SpeechStartEvent implements STTEvent {
 }
diff --git a/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/SpeechStopEvent.java b/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/SpeechStopEvent.java
index e858484436..d76cbe6d99 100644
--- a/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/SpeechStopEvent.java
+++ b/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/SpeechStopEvent.java
@@ -12,10 +12,13 @@
  */
 package org.openhab.core.voice;
 
+import org.eclipse.jdt.annotation.NonNullByDefault;
+
 /**
  * A {@link STTEvent} fired when the {@link STTService} stops hearing speech.
  *
  * @author Kelly Davis - Initial contribution
  */
+@NonNullByDefault
 public class SpeechStopEvent implements STTEvent {
 }
diff --git a/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/TTSException.java b/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/TTSException.java
index e4b66e4d29..39830e0cc8 100644
--- a/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/TTSException.java
+++ b/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/TTSException.java
@@ -12,11 +12,14 @@
  */
 package org.openhab.core.voice;
 
+import org.eclipse.jdt.annotation.NonNullByDefault;
+
 /**
  * General purpose TTS exception
  *
  * @author Kelly Davis - Initial contribution
  */
+@NonNullByDefault
 public class TTSException extends Exception {
 
     private static final long serialVersionUID = 1L;
diff --git a/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/TTSService.java b/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/TTSService.java
index 17949deb95..c80b86e773 100644
--- a/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/TTSService.java
+++ b/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/TTSService.java
@@ -15,6 +15,8 @@ package org.openhab.core.voice;
 import java.util.Locale;
 import java.util.Set;
 
+import org.eclipse.jdt.annotation.NonNullByDefault;
+import org.eclipse.jdt.annotation.Nullable;
 import org.openhab.core.audio.AudioFormat;
 import org.openhab.core.audio.AudioStream;
 
@@ -24,6 +26,7 @@ import org.openhab.core.audio.AudioStream;
  * @author Kelly Davis - Initial contribution
  * @author Kai Kreuzer - Refactored to use AudioStreams
  */
+@NonNullByDefault
 public interface TTSService {
 
     /**
@@ -39,7 +42,7 @@ public interface TTSService {
      * @param locale the locale to provide the label for
      * @return a localized string to be used in UIs
      */
-    public String getLabel(Locale locale);
+    public String getLabel(@Nullable Locale locale);
 
     /**
      * Obtain the voices available from this TTSService
diff --git a/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/Voice.java b/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/Voice.java
index 89cf198f0b..0d8bf72ad2 100644
--- a/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/Voice.java
+++ b/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/Voice.java
@@ -14,11 +14,14 @@ package org.openhab.core.voice;
 
 import java.util.Locale;
 
+import org.eclipse.jdt.annotation.NonNullByDefault;
+
 /**
  * This is the interface that a text-to-speech voice has to implement.
  *
  * @author Kelly Davis - Initial contribution
  */
+@NonNullByDefault
 public interface Voice {
 
     /**
diff --git a/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/internal/DialogProcessor.java b/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/internal/DialogProcessor.java
index 3de1d6541c..4a624784d3 100644
--- a/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/internal/DialogProcessor.java
+++ b/bundles/org.openhab.core.voice/src/main/java/org/openhab/core/voice/internal/DialogProcessor.java
@@ -12,8 +12,11 @@
  */
 package org.openhab.core.voice.internal;
 
+import static org.openhab.core.voice.internal.VoiceManagerImpl.getBestMatch;
+
 import java.util.HashSet;
 import java.util.Locale;
+import java.util.Set;
 
 import org.openhab.core.audio.AudioException;
 import org.openhab.core.audio.AudioFormat;
@@ -104,7 +107,7 @@ public class DialogProcessor implements KSListener, STTListener {
 
     public void start() {
         try {
-            ks.spot(this, source.getInputStream(format), locale, this.keyword);
+            ks.spot(this, source.getInputStream(format), locale, keyword);
         } catch (KSException e) {
             logger.error("Encountered error calling spot: {}", e.getMessage());
         } catch (AudioException e) {
@@ -113,10 +116,10 @@ public class DialogProcessor implements KSListener, STTListener {
     }
 
     private void toggleProcessing(boolean value) {
-        if (this.processing == value) {
+        if (processing == value) {
             return;
         }
-        this.processing = value;
+        processing = value;
         if (listeningItem != null && ItemUtil.isValidItemName(listeningItem)) {
             OnOffType command = (value) ? OnOffType.ON : OnOffType.OFF;
             eventPublisher.post(ItemEventFactory.createCommandEvent(listeningItem, command));
@@ -126,13 +129,12 @@ public class DialogProcessor implements KSListener, STTListener {
     @Override
     public void ksEventReceived(KSEvent ksEvent) {
         if (!processing) {
-            this.isSTTServerAborting = false;
+            isSTTServerAborting = false;
             if (ksEvent instanceof KSpottedEvent) {
                 toggleProcessing(true);
                 if (stt != null) {
                     try {
-                        this.sttServiceHandle = stt.recognize(this, source.getInputStream(format), this.locale,
-                                new HashSet<>());
+                        sttServiceHandle = stt.recognize(this, source.getInputStream(format), locale, new HashSet<>());
                     } catch (STTException e) {
                         say("Error during recognition: " + e.getMessage());
                     } catch (AudioException e) {
@@ -149,14 +151,14 @@ public class DialogProcessor implements KSListener, STTListener {
     @Override
     public synchronized void sttEventReceived(STTEvent sttEvent) {
         if (sttEvent instanceof SpeechRecognitionEvent) {
-            if (!this.isSTTServerAborting) {
-                this.sttServiceHandle.abort();
-                this.isSTTServerAborting = true;
+            if (!isSTTServerAborting) {
+                sttServiceHandle.abort();
+                isSTTServerAborting = true;
                 SpeechRecognitionEvent sre = (SpeechRecognitionEvent) sttEvent;
                 String question = sre.getTranscript();
                 try {
                     toggleProcessing(false);
-                    String answer = hli.interpret(this.locale, question);
+                    String answer = hli.interpret(locale, question);
                     if (answer != null) {
                         say(answer);
                     }
@@ -167,9 +169,9 @@ public class DialogProcessor implements KSListener, STTListener {
         } else if (sttEvent instanceof RecognitionStopEvent) {
             toggleProcessing(false);
         } else if (sttEvent instanceof SpeechRecognitionErrorEvent) {
-            if (!this.isSTTServerAborting) {
-                this.sttServiceHandle.abort();
-                this.isSTTServerAborting = true;
+            if (!isSTTServerAborting) {
+                sttServiceHandle.abort();
+                isSTTServerAborting = true;
                 toggleProcessing(false);
                 SpeechRecognitionErrorEvent sre = (SpeechRecognitionErrorEvent) sttEvent;
                 say("Encountered error: " + sre.getMessage());
@@ -186,15 +188,23 @@ public class DialogProcessor implements KSListener, STTListener {
         try {
             Voice voice = null;
             for (Voice currentVoice : tts.getAvailableVoices()) {
-                if (this.locale.getLanguage().equals(currentVoice.getLocale().getLanguage())) {
+                if (locale.getLanguage().equals(currentVoice.getLocale().getLanguage())) {
                     voice = currentVoice;
                     break;
                 }
             }
-            if (null == voice) {
+            if (voice == null) {
                 throw new TTSException("Unable to find a suitable voice");
             }
-            AudioStream audioStream = tts.synthesize(text, voice, null);
+
+            Set<AudioFormat> audioFormats = tts.getSupportedFormats();
+            AudioFormat audioFormat = getBestMatch(audioFormats, sink.getSupportedFormats());
+            if (audioFormat == null) {
+                throw new TTSException("No compatible audio format found for TTS '" + tts.getId() + "' and sink '"
+                        + sink.getId() + "'");
+            }
+
+            AudioStream audioStream = tts.synthesize(text, voice, audioFormat);
 
             if (sink.getSupportedStreams().stream().anyMatch(clazz -> clazz.isInstance(audioStream))) {
                 try {
diff --git a/bundles/org.openhab.core.voice/src/test/java/org/openhab/core/voice/STTExceptionTest.java b/bundles/org.openhab.core.voice/src/test/java/org/openhab/core/voice/STTExceptionTest.java
index 9c53f1f06f..086e897bba 100644
--- a/bundles/org.openhab.core.voice/src/test/java/org/openhab/core/voice/STTExceptionTest.java
+++ b/bundles/org.openhab.core.voice/src/test/java/org/openhab/core/voice/STTExceptionTest.java
@@ -14,6 +14,7 @@ package org.openhab.core.voice;
 
 import static org.junit.jupiter.api.Assertions.assertNotNull;
 
+import org.eclipse.jdt.annotation.NonNullByDefault;
 import org.junit.jupiter.api.Test;
 
 /**
@@ -21,6 +22,7 @@ import org.junit.jupiter.api.Test;
  *
  * @author Kelly Davis - Initial contribution
  */
+@NonNullByDefault
 public class STTExceptionTest {
 
     /**
diff --git a/bundles/org.openhab.core.voice/src/test/java/org/openhab/core/voice/SpeechRecognitionErrorEventTest.java b/bundles/org.openhab.core.voice/src/test/java/org/openhab/core/voice/SpeechRecognitionErrorEventTest.java
index 7e884d7183..d34c52afb7 100644
--- a/bundles/org.openhab.core.voice/src/test/java/org/openhab/core/voice/SpeechRecognitionErrorEventTest.java
+++ b/bundles/org.openhab.core.voice/src/test/java/org/openhab/core/voice/SpeechRecognitionErrorEventTest.java
@@ -14,6 +14,7 @@ package org.openhab.core.voice;
 
 import static org.junit.jupiter.api.Assertions.*;
 
+import org.eclipse.jdt.annotation.NonNullByDefault;
 import org.junit.jupiter.api.Test;
 
 /**
@@ -21,6 +22,7 @@ import org.junit.jupiter.api.Test;
  *
  * @author Kelly Davis - Initial contribution
  */
+@NonNullByDefault
 public class SpeechRecognitionErrorEventTest {
 
     /**
diff --git a/bundles/org.openhab.core.voice/src/test/java/org/openhab/core/voice/SpeechRecognitionEventTest.java b/bundles/org.openhab.core.voice/src/test/java/org/openhab/core/voice/SpeechRecognitionEventTest.java
index 0351cc785e..e28dd57f17 100644
--- a/bundles/org.openhab.core.voice/src/test/java/org/openhab/core/voice/SpeechRecognitionEventTest.java
+++ b/bundles/org.openhab.core.voice/src/test/java/org/openhab/core/voice/SpeechRecognitionEventTest.java
@@ -14,6 +14,7 @@ package org.openhab.core.voice;
 
 import static org.junit.jupiter.api.Assertions.*;
 
+import org.eclipse.jdt.annotation.NonNullByDefault;
 import org.junit.jupiter.api.Test;
 
 /**
@@ -21,6 +22,7 @@ import org.junit.jupiter.api.Test;
  *
  * @author Kelly Davis - Initial contribution
  */
+@NonNullByDefault
 public class SpeechRecognitionEventTest {
 
     /**
diff --git a/bundles/org.openhab.core.voice/src/test/java/org/openhab/core/voice/TTSExceptionTest.java b/bundles/org.openhab.core.voice/src/test/java/org/openhab/core/voice/TTSExceptionTest.java
index e2d6afb458..61f547154d 100644
--- a/bundles/org.openhab.core.voice/src/test/java/org/openhab/core/voice/TTSExceptionTest.java
+++ b/bundles/org.openhab.core.voice/src/test/java/org/openhab/core/voice/TTSExceptionTest.java
@@ -14,6 +14,7 @@ package org.openhab.core.voice;
 
 import static org.junit.jupiter.api.Assertions.assertNotNull;
 
+import org.eclipse.jdt.annotation.NonNullByDefault;
 import org.junit.jupiter.api.Test;
 
 /**
@@ -21,6 +22,7 @@ import org.junit.jupiter.api.Test;
  *
  * @author Kelly Davis - Initial contribution
  */
+@NonNullByDefault
 public class TTSExceptionTest {
 
     /**
diff --git a/itests/org.openhab.core.voice.tests/src/main/java/org/openhab/core/voice/internal/KSServiceStub.java b/itests/org.openhab.core.voice.tests/src/main/java/org/openhab/core/voice/internal/KSServiceStub.java
index f6dd6ae436..02709cbd49 100644
--- a/itests/org.openhab.core.voice.tests/src/main/java/org/openhab/core/voice/internal/KSServiceStub.java
+++ b/itests/org.openhab.core.voice.tests/src/main/java/org/openhab/core/voice/internal/KSServiceStub.java
@@ -54,7 +54,7 @@ public class KSServiceStub implements KSService {
 
     @Override
     public Set<Locale> getSupportedLocales() {
-        return null;
+        return Set.of();
     }
 
     @Override
diff --git a/itests/org.openhab.core.voice.tests/src/main/java/org/openhab/core/voice/internal/STTServiceStub.java b/itests/org.openhab.core.voice.tests/src/main/java/org/openhab/core/voice/internal/STTServiceStub.java
index 7027223156..4a60d4a841 100644
--- a/itests/org.openhab.core.voice.tests/src/main/java/org/openhab/core/voice/internal/STTServiceStub.java
+++ b/itests/org.openhab.core.voice.tests/src/main/java/org/openhab/core/voice/internal/STTServiceStub.java
@@ -15,6 +15,8 @@ package org.openhab.core.voice.internal;
 import java.util.Locale;
 import java.util.Set;
 
+import org.eclipse.jdt.annotation.NonNullByDefault;
+import org.eclipse.jdt.annotation.Nullable;
 import org.openhab.core.audio.AudioFormat;
 import org.openhab.core.audio.AudioStream;
 import org.openhab.core.voice.STTException;
@@ -28,6 +30,7 @@ import org.openhab.core.voice.STTServiceHandle;
  * @author Mihaela Memova - Initial contribution
  * @author Velin Yordanov - migrated from groovy to java
  */
+@NonNullByDefault
 public class STTServiceStub implements STTService {
 
     private static final Set<AudioFormat> SUPPORTED_FORMATS = Set.of(AudioFormat.MP3, AudioFormat.WAV);
@@ -41,13 +44,13 @@ public class STTServiceStub implements STTService {
     }
 
     @Override
-    public String getLabel(Locale locale) {
+    public String getLabel(@Nullable Locale locale) {
         return STTSERVICE_STUB_LABEL;
     }
 
     @Override
     public Set<Locale> getSupportedLocales() {
-        return null;
+        return Set.of();
     }
 
     @Override
diff --git a/itests/org.openhab.core.voice.tests/src/main/java/org/openhab/core/voice/internal/TTSServiceStub.java b/itests/org.openhab.core.voice.tests/src/main/java/org/openhab/core/voice/internal/TTSServiceStub.java
index b0a184d243..0142bb7788 100644
--- a/itests/org.openhab.core.voice.tests/src/main/java/org/openhab/core/voice/internal/TTSServiceStub.java
+++ b/itests/org.openhab.core.voice.tests/src/main/java/org/openhab/core/voice/internal/TTSServiceStub.java
@@ -14,10 +14,12 @@ package org.openhab.core.voice.internal;
 
 import java.io.IOException;
 import java.util.Collection;
-import java.util.HashSet;
 import java.util.Locale;
 import java.util.Set;
+import java.util.stream.Collectors;
 
+import org.eclipse.jdt.annotation.NonNullByDefault;
+import org.eclipse.jdt.annotation.Nullable;
 import org.openhab.core.audio.AudioFormat;
 import org.openhab.core.audio.AudioStream;
 import org.openhab.core.voice.TTSException;
@@ -33,6 +35,7 @@ import org.osgi.framework.ServiceReference;
  * @author Mihaela Memova - Initial contribution
  * @author Velin Yordanov - migrated from groovy to java
  */
+@NonNullByDefault
 public class TTSServiceStub implements TTSService {
 
     private static final Set<AudioFormat> SUPPORTED_FORMATS = Set.of(AudioFormat.MP3, AudioFormat.WAV);
@@ -40,45 +43,42 @@ public class TTSServiceStub implements TTSService {
     private static final String TTS_SERVICE_STUB_ID = "ttsServiceStubID";
     private static final String TTS_SERVICE_STUB_LABEL = "ttsServiceStubLabel";
 
-    private Set<Voice> availableVoices;
-    private BundleContext context;
+    private @Nullable BundleContext context;
+
+    public TTSServiceStub() {
+    }
 
     public TTSServiceStub(BundleContext context) {
         this.context = context;
     }
 
-    public TTSServiceStub() {
-    }
-
     @Override
     public String getId() {
         return TTS_SERVICE_STUB_ID;
     }
 
     @Override
-    public String getLabel(Locale locale) {
+    public String getLabel(@Nullable Locale locale) {
         return TTS_SERVICE_STUB_LABEL;
     }
 
     @Override
     public Set<Voice> getAvailableVoices() {
-        availableVoices = new HashSet<>();
-        Collection<ServiceReference<Voice>> refs;
+        BundleContext bundleContext = this.context;
+        if (bundleContext == null) {
+            return Set.of();
+        }
+
         try {
-            refs = context.getServiceReferences(Voice.class, null);
-            if (refs != null) {
-                for (ServiceReference<Voice> ref : refs) {
-                    Voice service = context.getService(ref);
-                    if (service.getUID().startsWith(getId())) {
-                        availableVoices.add(service);
-                    }
-                }
-            }
+            Collection<ServiceReference<Voice>> refs = bundleContext.getServiceReferences(Voice.class, null);
+            return refs.stream() //
+                    .map(ref -> bundleContext.getService(ref)) //
+                    .filter(service -> service.getUID().startsWith(getId())) //
+                    .collect(Collectors.toSet());
         } catch (InvalidSyntaxException e) {
             // If the specified filter contains an invalid filter expression that cannot be parsed.
-            return null;
+            return Set.of();
         }
-        return availableVoices;
     }
 
     @Override
diff --git a/itests/org.openhab.core.voice.tests/src/main/java/org/openhab/core/voice/internal/VoiceStub.java b/itests/org.openhab.core.voice.tests/src/main/java/org/openhab/core/voice/internal/VoiceStub.java
index aa5f0c3c4d..a5cd22a5a0 100644
--- a/itests/org.openhab.core.voice.tests/src/main/java/org/openhab/core/voice/internal/VoiceStub.java
+++ b/itests/org.openhab.core.voice.tests/src/main/java/org/openhab/core/voice/internal/VoiceStub.java
@@ -14,6 +14,7 @@ package org.openhab.core.voice.internal;
 
 import java.util.Locale;
 
+import org.eclipse.jdt.annotation.NonNullByDefault;
 import org.openhab.core.voice.Voice;
 
 /**
@@ -22,6 +23,7 @@ import org.openhab.core.voice.Voice;
  * @author Mihaela Memova - Initial contribution
  * @author Velin Yordanov - migrated from groovy to java
  */
+@NonNullByDefault
 public class VoiceStub implements Voice {
 
     private TTSServiceStub ttsService = new TTSServiceStub();