This would actually be the first step towards music mods for this game if we r able to unpack the music files.

the music files r inside this folder.
steamapps\common\FINAL FANTASY XV\datas\sound\resources\20000music\jp
the files r stored in .sab format. if we want all those Regalia custom music mods, then this is the first thing tht requires unpacking.
here there r tons of music files and I think the music tht plays inside Regalia r the files which begin with bgm_car.
These files r in .sab format, which I am unable to extract or play. used vgmstream, but didn't work. and to use quickbms I needs a proper script for the PC version.

and then there is another folder in the same resources folder called 20001music2 inside which the files r stored in .mab format. I think this folder mostly contains the common music in this game for example the ones we hear in the main menu etc. I am saying this on a guess.