Make automatic music volume adjustment smarter

[J. King]

Currently, the automatic music volume adjustment for voice is performed even when voice volume is set to zero.

Moreover, the music volume adjustment is performed even if the music volume is already set low; it may make more sense to set a floor for the adjustment, possibly relative to the speech volume setting.

Finally, it would be nice if the adjustment were entirely optional.

[Xwvm Art]

Change applied in development branch to avoid ducking briefing music, if the voice volume is more than 30% greater than the music volume.

[Xwvm Art]

Marking as resolved as this has been implemented.