Hiya. Welcome to TLS!
I'm very interested in the part with the music selection. There are some intriguing entries on there that don't seem familiar at all. I remember that someone said that the actual music sequencing data that each map uses is held within the map file itself (rather than the sound file where the instrument banks are).
This is true, the music is held within each field file. You can get the instrumental sounds elsewhere, but to rip the music you would need to know R3000 Assembly and understand the PSF format. I am yet to find anyone with the programming skill to do a music rip of all the field files in the original Japanese release. Only by doing this could we be sure that we've completely scoured the game for unused music tracks (not counting the minigames, of which I am not sure how their music could be ripped).
The BLACKBG7 text (Kazuhiko Yoshioka's debug room), especially the unflagged music selection track, is fascinating.
GlitterBerri's Debug Text entry
The final field only contains four music tracks, which are labelled as follows in Makou Reactor:
Music #0: Turk's Theme. This one is never triggered naturally in this field, as far as I know, but modding the field to ignore Music #1 will cause #0 to play instead.
Music #1: The Oppressed.
Music #2: Let The Battles Begin
Music #3: Mako Cannon - The Destruction of Shinra (portion played in FMV with Rufus).
EDIT: There *might* be more music tracks in this field. I tried out #4 and #7, but all I got was silence. I am not in the mood to try all the numbers though.
Either the field was once riddled with music, or the options were to transport you to the correct fields that would play those themes. We honestly do not know. In the tobal demo, there is no separate folder for music tracks (except, again, for the instrument sounds), so there probably was never a separate folder that the game was to refer to.
I also remember that one of the BLACKBG maps had a version of Aeris' theme that may/may not be different from the final version (you can tell that the incorrect instruments are playing, indicating that the song must've been around when the instrument bank was different).
Here is the music piece you mentioned, which I like to call "Aeris' Corrupt Theme".
So yes, the instrument bank must have been different.
This room is called BLACKBGA, and the author is Motomu Toriyama. You can tell that it is a beta version BLACKBG2, Motomu Toriyama's final debug room. Just why he switched from BGA to BG2, we can only guess.
You could use the early demo's instrument bank file to see if it sounds a bit better, but I don't know how this person managed to get this to load.
This is a f*cking excellent idea! Hopefully someone with the know-how can try this out.