http://www.vintagesynth.org/phpBB2/viewtopic.php?t=16830
contents of the synth chip:
1 quadrangle wave with sweep and envelope
1 quadrangle wave with envelope
1 noise sound generator
1 custom wave form…
Interesting?
When it talks about soundchip, it’s ALLWAYS interesting ;D
I’m assuming you’ve heard of nanoloop and Little Sound DJ?
I’m interested in the Gameboy as a MIDIBox LCD and SID-CS
I’d personally like an interface that plugs into the cartidge port and allows direct access to the sound channels via either MIDI or an MB hardware controller of some sort. Now that would be cool!
Everything you could ever want to know about the sound potion of the Gameboy processor (the Z80) is included in this document.
At least it has more than I could bother to read…
I know, I know, again with the bumping…
I cracked open a Gameboy Pocket, and it’s pretty amazing, all the pins have holes for headers, and pretty much everything’s labeled. I see Sin, Sout, Sck lines on the processor, MA0-MA11, MD0-MD7, SO1-SO2, LD0-LD1, CPL, CPG, CP, ST, S, FR, etc.
And of course, audio out, a DC-DC converter, and a pot for volume 
I know most folks have been focusing on external connections to gameboys, but I figured the whole ‘internal’ approach might yield better results.
Edit: From my research, it seems that the Z80 would require a cartridge to be programmed. These carts and programmers are getting harder to find… Mind you, the GB Advance/GBA SP/GB DS have better cores, and let you access the serial port directly.
As much as I love the Z80 pseudo-sound-chip, you can get GBA’s for $10 or $20 on ebay. I’m thinking it might be easier to just develop with those :-[