Conn a friend of mine, with collaboration with another smw hacker made cd quality music possible via the msu1 chip. This chip can run via an emulator or on real hardware with the sd2snes cart which has this chip.
You have to convert the mp3s to pcm files to make them work with msu1.
Here is a link as well as other info regarding this.
If you have custom music that you have made in mp3 format, you would have to either port over your songs to the standard snes sound engine by remaking the songs or convert them to pcm files and use the msu1.
Not a lot of people here have used the msu1 for music, but really in my opinion this is the future of having custom music in snes games. You could also use the msu1 for sound effects instead of music kind of like how in (GBA) smw advanced mario has voices, you could do that via msu1 and there is a hack which already does this called Yoshi's Strange Quest if you want to check it out.