Using the list on the wiki as reference, here's my own extra observations:
- 20.adx - Deep Breath Deep Breath - used on the first full-moon mission, and during Operation Babehunt
- Duplicates: 23, 24 - I don't believe DBDB is used anywhere else, and I have not heard other versions (ie. calls not for 20.adx) play.
- 22.adx - A Deep Mentality - used during the Love Hotel mission
- Duplicates: 76 - As above, appears unused
- 43.adx - Joy - used during cutscenes and many social links.
- Duplicates: 38 - curiously, this one is earlier numerically. It doesn't however appear to be used. I thought it could be a cutscene or even certain-social-links alternate (in case Atlus decided to create variations), but it hasn't turned up in testing. Actually turns out it is used, in the very least during events on 11/4 (post-Hanged Man party) and 11/9 (Ryoji at school). It isn't used when Junpei first joins SEES, which was what made me presume it wasn't a cutscene version.
- 60.adx - After The Battle - used for, well, battle victory
- Duplicates: 78.adx - this one is used. This is used during shuffle time, despite being otherwise exactly the same.
- 52.adx - Memories of the School, also known as "Memories of the Garden" apparently?, that plays as the third school theme.
- 88.adx - Memories of the School as it appears on the soundtrack, that plays during the final scene in the ending.
- 82 to 84.adx - These "ambience" tracks to me sound like beta versions of The Voice Someone Calls, that progressively gains more elements. I personally suspect the hub music was going to evolve like Tartarus' main theme. These are probably unused though.
- 93.adx - The "Velvet Room Interlude" plays during the ending when the protagonist receives the voices of his social links.
- We can probably assume anything after 115.adx (Brand New Days) is WIP junk (... and sound effects and persona fusion), and no modders would need messing with them. I have always found it fascinating how Changing Seasons transitions into Master of Tartarus though...
(EDIT: oops maybe this would've been better in the modding guides forum)