core/homeassistant/components/keyboard/icons.json

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