I originally added it to IPlayer but it really doesn't apply to the remote audio player.  I also thought of adding setVolume to the IPlayer interface, and implementing ducking directly in AudioPlayerWithEvents, but I didn't want to lock us into one method of ducking (Spotify, for example, pauses when it ducks...).  I kinda ike separating specific functionality into specific interfaces, and letting the object being injected implement what it needs to accomplish its job.