IIRC, you will find a music.txt file in the assets/sound folder, which contains references to music pieces stored in the file sfcmusic.zip in the same folder. the filestructure describes which portion of music is played in what game sitution. So, all you have to do*, is to include your musics in the zip and modify the music.txt to play always your files by replaceing the previous references stored there. take care to match the soundformat used in the sfcmusic.zip when inserting your files, or the game will not be able to play them (yes, you have save your music in the format sfc uses there).
(* Sounds easy, doesn't it? A quick look in the music.txt file will show you, why there are so many Music-Mods around... Good luck anyway. )