core/homeassistant/components/tts/icons.json

{
  "entity_component": {
    "_": {
      "default": "mdi:speaker-message"
    }
  },
  "services": {
    "clear_cache": "mdi:delete",
    "say": "mdi:speaker-message",
    "speak": "mdi:speaker-message"
  }
}