core/homeassistant/components/tts/icons.json

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