Is it just midi? Did the Gameboy come with its own default sound samples?
If you are looking for something that's real and actually from the games, you want music rips. These include NSF files (NES), GBS files (Game Boy and Game Boy Color), VGM files (Master System and Genesis), GSF files (Game Boy Advance), USF files (Nintendo 64), PSF files (Playstation), and several others.