Support for per-input volume control

Hi,

I wanted to be able to edit the relative volumes of the incoming pcm streams and added basic volume adjustments + modified the example to make this work. 

I'm unsure about the _acoustic_ correctness of the way I've made this work - but it preliminarily works well with the example. (up/down to adjust example0.mp3 volume, left/right to adjust example1.mp3 volume). Do you mind taking a look?