http://en.wikipedia.org/wiki/ROM_hacking

I do have a problem with this being uncommon!:

Music hacking

Music hacks are relatively rare, due to the wide variety of ways games store music data (hence the difficulty in locating and modifying this data) and the difficulties in composing new music (or porting music from another game). As music cracking is very uncommon, many hacks do not have any ported/composed music added in. The program SapTapper can be used to hack Gameboy Advance music data, however, as many Gameboy Advance games use the M4A Engine (also called "Sappy Driver") for music.

The most common form of hacking music is porting music from one ROM file to another, usually both made by the same company. Porting music from two files will almost always pose some type of problem, as the format is almost always different.