I wasn't familiar with the Teensy Audio Library, it looks really fun and well-featured. Faust is also popular for this purpose (this is mentioned in the linked

I wasn't familiar with the Teensy Audio Library, it looks really fun and well-featured. Faust is also popular for this purpose (this is mentioned in the linked page). You write audio code in Faust and it gets compiled to tight C code, so that works well for embedding. It also has an impressive set of libraries. For a different approach, you could also try Soundpipe. It's a C library, not initially meant for embedding but it's small so it could work.