Steve Mostovoy and his partner Tim Boy Jr, worked for over 9 months to  create a TouchSynth and share all the documentation about it.

It was hard but they learnt  a lot about how not to organize code, how to think in terms of data, and how many amazing things you can do on a 16MHz processor:

This project uses an Arduino Mega, a resistive touchpad, and an 8×8 LED matrix to create a simple digital audio workstation and synthesizer. You can compose and play songs in real time and use different types of sounds. It was a nine month process between me and my partner during our senior year of high school to create this. The result is a really nifty Arduino project that showcases the flexibility of Arduino devices, and it sounds good as well!

Take a look at the video to see how it works: