According to the Game Boy schematic:
The cart edge pin 31 "VIN (or Voice IN)" routes to the "CPU" pin 61, which most likely goes through a mixer, and out L and R (audio) OUT pins.
This means that in potential it's very possible to develop an FPGA/CPLD-based Game Boy flash cart with additional expansion audio. More or less since the source is open, Nintendo PowerPak logic could be copy/pasted with a couple tweaks and VRC6/"MMC6"/etc. could be added as additional sound channels coming in from the flash cart, to the "CPU" and out the speakers/headphones jack. The only downfall is that expansion audio would have to be monophonic.
Food for thought for all you potential flash cart developers.
Also, LSDJ could potentially have new toys to play with.
