So a while back, I decided to make my own Game Boy sound driver, mainly due to a lack of decent GB sound tools with good feature sets. Deflemask got pretty close in my book, but the problem is it can't easily be used for homebrew. My main goal for DevSound was to create a sound driver which could be used for homebrew. Currently it supports both hardware and software envelopes (even simultaneously!), arpeggios, pulse width modulation, and up to 128 waves for CH3 (along with programmable wave phasing). It currently does not support any form of pitch manipulation, but that's on my todo list.
Grab the source here: https://github.com/DevEd2/DevSound
Demo ROM is attached, as well as a demo GBS file.
EDIT: I forgot to mention that I specifically designed it so Deflemask modules would be easy to convert. Eventually I may make a DMF-to-DevSound converter... but now is not that time.