Avoid dirty play()/stop() trick

Before this, the audioTrack was stopped between each packet. If some
applications were listening to this event and logged something, it
flooded the logs and degraded the audio communication.

This commits stop the audioTrack only when the mixer has no input
anymore.
Do not keep the mixer lock while waiting

Use Object.wait(delay) instead of Thread.sleep(delay).
Avoid some buffer underruns.