I actually experimented with this using ultrasonic signals (most people can't hear above ~16-17kHz), and with very simple modulation was able to transfer ~30bps. Was a very fun proof of concept, using just a cell phone and a speaker! I think the idea is great and there are many applications for it.

In this one I embedded the signal into a song, and then the phones would parse the modulated data to all synchronize together and create an ad hoc light show with the flashlights!