I don't think this matters too much -- QString::operator== exits early if the strings are of different length anyway. It would be different if this were a per-sample loop or something but this a for-loop with a body that does a heavy function call (```process()```) so it's really not that hot.

I would prefer an "isCompressorSidechain()" on EngineChannel that returns false for all but EngineMicrophone. dynamic_cast is IMO a hack (relies on Mixxx being built with RTTI) and should only be used as a last resort.

We will have multiple microphone support in the future so we shouldn't hard-code a single key here also.