Audio Mixer: move Static variable to a member variable.

The two instances of this class were sharing a frac variable causing
audio glitches when both were running (which is now all the time).

Fixes issue 7463 (Since DTK merge, audio has staic in it).
Correct left/right volume for padded samples.
Merge pull request #639 from phire/fix-audio-issues

Fix audio issues since DTK merge.