Add an opt-in setting for broadcasting your microphone in stereo

Currently, all voice (input) audio is sent by the Discord Client as mono streams, which makes perfect sense in the majority of use cases, most microphones are mono, and even worse many audio interfaces force the microphone to go through only one of two channels.

However, there are times when a user wants to set their input device to something like Stereo Mix, or even the uncommon stereo binaural microphone for ASMR. In those cases, having a checkbox in Voice & Video to allow opt-in switching from mono to stereo voice chat transmission would be useful.