I'm really confused now because I thought Low-Gain invented the pro sound mod!! xD
This looks really cool though, hopefully you can do some fun stuff with it! Maybe the wasted address space that was used for the mirrored RAM that nitro2k01 was talking about could be used for an extra soundchip instead (ps. I have no idea if this is how GB/Z80 works)? That would be quite cool (although new software would have to be created I guess).
It looks fun anyway. Good luck!