Made a simple theremin in vrchat udon:
https://streamable.com/r9iekm
udonsharp code: https://gist.github.com/hiinaspace/ec7784c5812cc67e701a14ce3343b21b

Unfortunately audio synthesis is hamstrung by the fact that the udon VM is about 100000x slower than the equivalent C# code, so you can maybe synthesize about 500 samples each frame before you drop frames (with exactly one person in the room), and that's just a single sin wave. Cool that they let you synthesize audio from raw samples at all though.