configurable audio sample rate and channel count

This PR allows people to override the default sample rate (44100.0) and channel count (2).

It required quite a lot of little changes to make it all hold together. *Hopefully I didn't miss any.* I tested it on the SampleBroadcaster application and it all seems to work.

**NOTE:** I did have to uncomment a few lines around the audio stuff in `VCSimpleSession.mm` in order to hear anything in the sample application. I'm guessing those were commented-out by mistake?