[gfwilliams]

There's been info on doing this with Espruino for about a year now: http://www.espruino.com/Headphone. There's even an online-only version of the Web IDE that does two-way communications: http://espruino.github.io/EspruinoWebIDE

With 3.3v logic, it turns out you don't even need the transistor - just a capacitor and bias resistors.