So I added stereo sound, by using a small delay and volume change to make each of the two pulse wave channels sound like they're coming from opposite sides. I left the triangle wave in the center (there's only one and it's usually bass notes) and I also made the noise channel stereo. I mostly can't notice a difference unless I'm wearing headphones, so listen with headphones.

I tried to make a Windows version of the emulator, but the documentation on how to use GLFW with MinGW isn't really there, and so even though I've done it before with MME, I didn't get far. I might try again later but it's not looking too hopeful. In particular MinGW was giving me a bunch of errors in its own header files and I can't fix that.

For comparison, here's what it previously sounded like:

...and here's what FCEUX sounds like: