Daniel Rapp is a developer who builds small browser demos in signal and image processing, including Doppler motion sensing and spectrogram images.
HN links it for the Doppler motion sensing demo, which a commenter says is simpler than FingerIO but still interesting, and for the spectroface demo.
You may be interested in https://github.com/quiet/quiet-js and https://danielrapp.github.io/spectroface/, both encode text / images over audible frequencies, but of course they sound weird.
However it is technically trivial to encode "hidden" data at inaudible frequencies (>25 kHz) along with a song/whatever in the audible range and recover the data with a high pass filter. You would have to explain, however, the need for >48kHz sample rate.
Another, stealthier option would be to encode information in the background noise of a recording, but this would require more complex encoding/filtering techniques.
This is not same as FingerIO since it does not uses sophisticated signal processing but still interesting.
Make sure that you remove earphones before using it.
https://danielrapp.github.io/doppler/