DIY synth building and hacking is increasingly popular. The PISound interface from Blokas aims to provide you with pro ins and outs for your Raspberry Pi system.
Blokas have created pisound, an ultra-low latency high-quality sound card and MIDI interface for Raspberry Pi which you can even run Pure Data patches from.
Joshua Curry has created a Raspberry Pi synthesizer, named Blue Order. It’s menu system has been written in Python mainly based from example script that came with the Displayotron Hat display.
piStudio brings back the 90s low-fi recording four-track! Inspired by old Tascam four-tracks and powered by the incredibly cool Raspberry Pi ZERO. It's full of excellent features.