Put a teensy into a cartridge as a whole Gameboy to play sounds? Dude, that's insane - I love it! Let me know if I can assist you with getting a Teensy Gameboy emulator up and running!

You should probaby know, though, that sound implementation on my emulator isn't as accurate as it should be. This is because otherwise I'd get framerate drops. This could probably be improved if you cut back on the graphics engine and treat audio with priority.