[Stewart] tipped us about his very nice project: pokewithastick. It is an Arduino compatible board (hardware, not footprint) based on the ATMEGA1284P which can be programmed to collect and post dat… ...

They used the webaudio API to send serial data to a remote Arduino. The hack uses a FET, a capacitor, and a few resistors. They didn’t quite build a real modem with the audio.