Reverse-engineering the Final Fantasy: Theatrhythm track format
gitlab.com
gitlab.com
If you’ve never engaged in this kind of reverse engineering, I’d recommend it. It’s fun. It feels a lot like sorcery once you succeed, and that can be pretty rewarding.
There was also a (now defunct) iOS game, which notably had a "Compose Scores" mode to edit your own tracks (though you couldn't change the BGM). I would be curious to see what the editor interface looked like.
Now, if the post were to say "the cartridge was protected, so I used hardware X to dump it, and software Y to decrypt it, here are instructions" that'd be circumvention description and thus subject to DMCA.
(disclaimer: I am not a lawyer)