Added a way to stop recording audio from input device without disabling voice chat + other improvements to input/output volume

I added the CVar voice_muteself so that one can stop recording from their input device without having to disable voice_enable, or having to switch to push-to-talk (which is still recording audio in the background, just not transmitting it). It should be easy for one to create a bind that toggles this CVar at the push of a button.