The first question is, are you sure someone hasn't done this already? There are repositories of GB music out there on the Internet, for use with specific media player plugins.
There's certainly nothing resembling "music files" in a GB ROM. As I understand it, there is no straightforward way to make a GB music dump - the dumps you can find out there basically consist of the ROM manually stripped down to just its music code. To convert that to .mid would require further reverse-engineering of the music code, and good understanding of the structure of the .mid format itself.
Topic: How do i rip Music from a GBC game (and convert it to the .mid format)? (Read 1499 times)