I've had the "pleasure" of trying to transmit data / timecodes through active speakers with a lot of signal processing.

I don't imagine any QAM-like stuff working. FSK does, but the transitions get a little wonky, and the different frequencies will often have different phase shifts. ASK modulation should work, but haven't gone there yet.

But its interesting. I know a thing or two (literary) about encoding in the radio spectrum, but none of the encodings I've found deals with systems that are so un-lineary.