play
Starts playing the speech queue from the beginning.
Items are spoken sequentially. Use onQueueProgress to track progress and onQueueFinished to be notified when all items have been spoken.
If a queue is already playing, it is restarted from the first item.
Return
SpeechResult.Success if playback started, SpeechResult.NotReady if the engine is not initialized, or SpeechResult.Error if the queue is empty.