the most basic explanation possible is that they're snes music files. you can make them yourself if you use zsnes or snes9x and use the option to "save spc". i think that that creates the sound file for the area you're in, so to make a full set, you would have to play through the whole game, or to all the areas with different music.
i really don't know much about them, sorry. maybe someone can provide a link with a little more detail? i wouldn't mind reading more myself.