Not sure if this is what you meant, but Chromecast's guest mode will pair using ultrasound. The guy who worked on it has some code using the WebAudio API on github and his linked blog:
No there was an article on here several months ago that I couldn't find when searching where someone wrote a PoC for the transmission of data via near-ultrasonic sounds from a computer speaker and microphone. I don't think it was this [1] one, but it shows the concept being used.
Maybe I'm misintepreting the OP's work and not realising how it differs in some subtle way? I get that they've put TCP/UDP over it, but is there something else I'm oblivious to?
https://github.com/borismus/sonicnet.js