Inspired by yesterday’s LED candle with a hidden song, I set up an arduino to transmit 16 different notes on 16 LEDs simultaneously. (The notes are laid out like the bass keys of an accordion, though you can’t really tell.)
The optical receiver hears all the notes at once, unless you cover some of them up.
Bonus: the flickering LEDs do crazy things to a cell phone camera.
The underlying idea and the arduino code (available here) are nearly identical to the Jeltone from a few years ago, just transmitting the sound through light instead of skin conduction.