THe Raspi CM is also used in some synthesizers like the Korg Wavestate or the Ableton Move.
As far as I can tell, there's no custom "firmware" to speak of, it's a patched Linux kernel running a kernelmode audio synthesis engine and a usermode app for the UI
https://www.reddit.com/r/ableton/comments/1fzd9ln/move_is_a_...