An audio version of this sort of thing was Chirp. I wrote the original audio code to implement something that sounded aesthetically pleasing like a bird twitter

An audio version of this sort of thing was Chirp. I wrote the original audio code to implement something that sounded aesthetically pleasing like a bird twitter, but actually used multi-carrier FM plus error correction and a whole bunch of stuff to be a "constellation" modem. I believe the company are still going. IIRC, when I left they'd got to the point of realising that in a noisy environment, even with echo cancellation, you couldn't really do serious data transfer - and why would you if you have Bluetooth anyway. The real application for these "last meter" contactless ideas that use stalwart built-in transducers like mic+speaker or LED+camera, are to send short codes, such as shortened URLs. There's a whole bunch of fun to had using them for surreptitious advertising and public steganography.