That's what I bought the first time, but unfortunately there don't seem to be good audio libraries for the Feather line, and the Teensy Audio library looks quite sophisticated. Or did I miss something?

They should be compatible. It's all Wiring at the end of the day. But IDK, I just write my own.

Which processor, though? There are a few.

I don't know if they've made something more updated, but I usually get the Feather M0 nrf52. The Bluetooth connectivity is super cool for controlling things, and as long as you don't enable encryption, it is relatively low latency.

I see, thanks! I'm going to get a few to play with, I've seen other people recommend that microcontroller too.