Michael Bauer, maker of the DIY REMI electronic wind instrument, has just published all the design and build notes from a pretty neat Samd21 based synth, designed for an Adafruit Itsy-Bitsy M0.

There is a very comprehensive build guide and all code is for the Arduino environment and up on GitHub.

Read more here: https://www.mjbauer.biz/Sigma6_M0_synth_weblog.htm

I always learn a lot from his writeups :)