The Teensy Audio Library is pretty amazing. https://www.pjrc.com/teensy/td_libs_Audio.html 

It allows for 16-bit 44.1kHz audio from a microcontroller. I'm using it to develop a synth onboard that doesn't make you want to cringe.

Here's the block diagram:

The hardest part of the synthesizer is the effort to smooth out the amplitude changes and pitch bends. Take a listen to these samples:

https://soundcloud.com/jeff-m-hopkins/open-woodwind-project-onboard-synth-demo-teensy-library

https://soundcloud.com/jeff-m-hopkins/we-three-kings

All sound is produced onboard the synth, including the reverb. I'm still actively developing the code base, but so far so good!