After wrapping up some SMS issues, I went back and implemented a bunch of missing Game Boy PAPU audio features. Channels are now properly emulated (squares, wave, and noise) along with length counter handling, volume envelope, and frequency sweep. Debugging some of these was tricky, but the result was worth it! This work has been integrated into master, and can be tested either standalone, or via libretro frontends.
Emux is now part of the libretro ecosystem (cloned at https://github.com/libretro/emux), and automatically built for multiple architectures and operating systems (so far Linux, Windows, and Android). RetroArch's core updater can automatically fetch the latest binaries of all supported machines, so it is definitely the easiest/best place to start if you want to test out this on-going project.