Earlier quoted context omitted.
> bitrate of 20 bits/s I can imagine this being used as a control protocol for home/building automation, 20 bits is quite a lot if you consider each bit a device being turned on or off only. I still don't know why would anyone do this instead of wifi, but definitely way cooler. Also, anyone visits you the first time, there's a guaranteed 20 minute talk about the setup :)
Would make for a pretty stealthy bootstrapping protocol to share things like wifi keys between devices or to do keystroke logging.
speaking of stealth, there was this (hypothetical?) malware from 2013 which spread via computer speakers: https://en.wikipedia.org/wiki/BadBIOS