Well, you can see that whatever I got couldn't be a .ogg or anything like that since it's only 20KB or so for a whole song. And different songs are different sizes, so I don't think it's likely that the whole file is itself a key into some other data structure. It must be something that translates into a MIDI or the equivalent, but I don't see any way of figuring it out with the information I have.
PS: you must have been really good if they let you win with a Gackt song
PS: you must have been really good if they let you win with a Gackt song