This patch has been floating around since the creation of An SMW Central Production, and I feel that now is a great time to add it to the database proper for the convenience of anyone wishing to use it.
What this patch does is, as the name implies, allow you to have multiple music tracks playing on the main Overworld map. This can be very useful if multiple world themes are present, since one track may not fit them all. To set songs, scroll to the bottom of the .asm file and edit the table. Each entry ($XX) represents the song for that particular 32x32 box on the map.
So what would I do to add custom music to Choc. Island?
Here, you can use this diagram to help you find the coordinates for Chocolate Island (in fact, if anyone here is on the fence about using this patch, this diagram should help you understand the coordinates better).
It's definitively a useful patch if you want not just graphical variety in the main map (and also because not every music is fitting if you have multiple themes on the main map).
The usage is a bit difficult, though, but that's the side effect from the lack of an overworld editor which supports this. Just remember to read the position in the bottom left corner in Lunar Magic to get the correct position (it has to be a multiple of four i.e. the last digit is either 0, 4, 8 or C so round the value down if it isn't the correct digit).