core/homeassistant/components/keyboard/icons.json

{
  "services": {
    "volume_up": "mdi:volume-high",
    "volume_down": "mdi:volume-low",
    "volume_mute": "mdi:volume-off",
    "media_play_pause": "mdi:play-pause",
    "media_next_track": "mdi:skip-next",
    "media_prev_track": "mdi:skip-previous"
  }
}