Oh this is really cool! Reminds me of ggwave [0].
It feels difficult to create hobbyist peripherals that interface with ones' phone -- trying to get cross-platform credentials to plug your own Arduino in via USB or connect via Bluetooth feels like a chore. I like the idea of phones communicating via some sort of audio library (ultrasonic maybe?) -- like R2-D2 chirping back and forth to communicate with other droids. I think this sort of thing could be part of a nice network of cross-device communication.