So the idea came to me from nowhere but I realised that Isaac and Castlevania: Symphony of the Night have some settings in common. Here's how I replaced the tracks:

Floors:
-Title screen -> Prayer
-Basement -> Dracula's Castle
-Cellar -> Final Toccata
-Caves -> Crystal Teardrops (I will maybe use it for flooded cave when afterbirth is released)
-Catacombs -> Lost Paintings
-Depths -> Abandonned pit
-Necropolis -> Rainbow Cemetary
-Womb/Utero -> Heavenly Doorway
-Cathedral -> Requiem for the gods
-Chest -> Prologue
(haven't done sheol and dark room for now)

Boss Battles:
-Normal Boss Battle -> Festival of Servants
-Challenge Room -> Death Ballad
-Mom's Foot -> Enchanted Banquet
-It lives/Isaac's heart -> Blood Relations
-Isaac -> Guardian
-??? Fight -> Dance of Illusions
(as for the floors, i haven't found anything for Satan and The Lamb for now)

Special Rooms:
-Shops -> Shop music from SOTN (it doesn't have a name)
-Arcades -> Vampire Killer (Castlevania 1 NES version)
-Secret Rooms -> Dance of Pales / Waltz of Perls
-Library -> Wood Carving Partita
(haven't found anything special for god or devil rooms)

Please note that I looped the tracks from the original soundtrack myself. Some little imperfections may appear here and there. Just report them to me if you notice them

I'm also thinking about modifying some jingles/sfx, like for example having the "HP UP" apparition SFX from SOTN playing once a boss fall.

Any other suggestion is welcome!

To install this mod, just copy/move the content of the zip into your "ressources" folder.