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.

Arduino 8 to GBA SO Arduino 9 to GBA SI Arduino 10 to GBA SD Arduino 11 to GBA SC That’s it, follow the README in his source code package and you’re on your way to some ARM development.