I learned something interesting. Turns out that you can use this method to switch out decrypted DLC Music .edat's(Provided in
this thread) with your own custom music of choice. Seems easier and less risky than trying to dig through the actual game .iso.
You follow pretty much everything the same way in the tutorial up until you finish making your .wav/.at3 music file, then you just have to rename it as the appropriate .edat file. Simple as that.
Admittedly, the hard part is figuring out which .edat's are the music files and which ones aren't. If you can open up the .edat's in something like Notepad, or better yet, a hex editor, and see the words "RIFF" and "WAVEfmt" in the first line, you can rest assured it's one of the actual DLC Music files. As for which files correspond to the actual DLC music tracks that appear on the BGM menu before battle... That's more of a guessing game.
As a word of warning, I would caution against people trying to do this with DLC fresh off of PSN. I haven't tried it, but I doubt it'd work. Worst case scenario, you'd get a game freeze.