Fixed stereo input and comparison

* Finally figured out how to get stereo input. Turned out the master constraint echoCancellation=false was all that was needed.
* testInputVolume() now returns the calculated db or volume for the provided buffer and channel.
* Rounded the db value as we do not more accuracy than a float without decimals.
Mono microphone implies same audio DB on both channels.. but the opposite is not true (you could just be incredibly lucky and get the same DB, no?)

Detection of a mono microphone being feed into the 2 channels isn't better detect by comparing the actual samples?