remove support for layered sound, clean up & debug volume control

@AmbientOSX: As a general design principle, poking at the inner workings of some other class is usually bad - and this is a good example of the reason why.  The code here was not integrated with the existing setVolume() function, so if anything had called that function it would have overridden this.

In general, try to have a single path through the code, avoid duplication, and encapsulate the implementation details in the class responsible for them....