Volume Jump Fix

Building on the previous fix for the audio volume jumps experienced
when switching tracks, this adds a double check that the underlying audio
layer has actually set the volume to the new value. This may fail because
the COMXAudio object it not yet initialised.

If there is a failure, then the volume change is retried in the next
iteration of COMXPlayer::Process().

*After running this for a few days on my Raspberry Pi, I have not experience the volume jump bug when playback completes a track, and switches to the next track.*