In order to make a music pack, you'll need a lump editor (such as Slade or XWE) and some music, preferably in .ogg format. You'll need to put the music in the lump editor and rename it to the music you want to replace. For example, if I wanted to replace Pipe Tower's underground music, I'd rename the new file to O_MARIO2.ogg and then import it into a new wad.