From the book ARDUINO FOR MUSICANS is the Stella Synthesizer project. It is a synthesizer based on an Arduino and the Mozzi sound library.
Built into a wooden cabinet, it is a simple and quickly assembled project.
I created the circuit diagram with ExpressSCH: StellaSynth.
I also made a board with ExpressPCB: StellaSynth.
As always there is also a front panel in addition, which I created as always with the Front Panel Designer: Stella Synth.