Ingame music customisation

[Enkidoh]

Given how music is used in this game, a full customization ability for actual BGM is quite frankly an impossibility (zone tracks that are cued to only play at certain times of the day or in certain parts of a zone, plus the default battle theme is separately cued if you finish a battle and then start another one quickly enough for instance). More likely you'll find past FF game music appearing as orchestrion rolls in the future, although again SE stressed it's 'not so easy to add them' as it seems, it is possible that it may be added as purchasable items (presumably on the Mogstation):

Adding songs from other FINAL FANTASY games
It's difficult for us to add songs that are not used in FFXIV, but it might be possible to add these in as optional items.

It can be done, although it's fairly convoluted.

(link removed)

Uh oh... you... might want to remove that link as it's pretty much hacking the game files which is forbidden under the ToS (including discussion of it here on the Forum).