Don’t confuse Audunios with Arduino in this case. Audunios is the name of a small do-it-yourself synthesizer-kit, that was invented and constructed by tinker.it, licenced under a open-source licence. Some home-made tinkering can be done with that, resulting in a self-cooked synth, ready to go. This version here was done by Denkitribe. The linernotes on YouTube tell about this fantastic work:

I build one and plugged a stylus controller which was ripped from Gakken SX-150. It works very well. Its sound is dry and crispy, but I think it is not bad for $50. Want one? Do It Yourself!

Beautiful. Here is another one, made by machinecollective:

Get all source-codes, how-to’s and more examples at the Audunios page.