Hi y'all! I poked around and couldn't find any projects that had done this so I figured I'd ask. I'd like to change the Gameboy Color boot sound (the little chime) into something else. I would probably include a sound chip (like from one of those singing birthday cards) that would activate on startup and disable the original sound. However, I'm not really sure how to do that (I couldn't even find pinouts for the GBC) and most of my mods have been xbox 360 controllers, so I'm not even used to working with handhelds/Nintendo hardware, so your thoughts/suggestions would be much appreciated.

I'm 90% sure that it's built into the BIOS, and therefore impossible.
That is, unless you want to recode the BIOS, fix the pointers that got screwed up when you reprogrammed it, get a flash memory chip for it, make it compatible, and then reflash the BIOS.