So I've been messing around trying to edit the music in MM2 using an assortment of guides and tools and I, being a the complete beginner that I am, try to find a simpler method than just changing random values in the .nsf and hoping for the best. Being familiar with Famitracker, I wondered if there was any way to import nsf files into the program, and sure enough some guy made a modified version which is located right here: http://famitracker.com/forum/posts.php?id=2284&sort=1
(editing is disabled on this version so you have to use the official version to edit).
So I loaded it up, messed around with the title theme a little bit, and exported the result as an nsf and loaded it and the original nsf into translhextion for a comparison. Immediately, I noticed that a large chunk of the substance of the original file was gone, replaced with random repeating digits. This was probably due to the fact that Famitracker can only open 1 "song" from an nsf file at one time, overwriting the space that the other songs occupied with garbage data.
Now I have tried scrolling through, trying to find bits that look like music, copying and pasting into the Hex editor in FCEUX, and nothing of note has happened besides game corruption, but I feel like I've barely scratched the surface. Either I'm just dumb and Famitracker has reformatted the nsf to where it's basically useless, or I have no idea what I'm doing or where I'm putting the code or what code I'm putting in so the desired result has still yet to be achieved. If you too are desperate for edited music in your game then I think it would be a good idea to test around with this (for science!). On the other had, if you have a lot of knowledge in this area and are ready to tell me that I'm way off, please do.