This mod replaces all of Dark Souls 3's soundtracks with Bloodborne music, except for the firelink shrine "betrayal" alternate theme.
While choosing which music to use, i tried to find the one that either sounds like the boss theme, design or order/placement inside the game. For instance, deacons of the deep is a boss with many enemies at once, just like celestial emissary. Vordt is a fast, flailing, quadruped early-game boss, like blood starved beast. Midir surely isnt a multi-enemy boss, but Living Failures' soundtrack is grandiose and epic, so i thought it would fit his boss fight. Obviously not every music will be perfect-matching, but i tried my best to find out which ones would fit most adequatly. 

List of Music Replacements:
Iudex Gundyr - Father Gascoigne
Firelink Shrine - Moonlit Melody (Hunter's Dream)
Vordt of the Boreal Valley - Blood Starved Beast
Curse Rotted Greatwood - Cleric Beast
Crystal Sage - Witch of Hemwick
Abyss Watchers - Watchers(Shadows of Yharnam)
Deacons of the Deep - Celestial Emissary
High Lord Wolnir - Rom, the Vacuous Spider
Old Demon King - Terror(Watchdog of the Old Lords and other chalice dungeon bosses)
Pontiff Sulyvahn - Queen of the Vilebloods(Martyr Logarius)
Aldritch - Ebrietas, Daughter of the Cosmos
Yhorm - One Reborn
Dancer of the Boreal Valley - Of the Pthumerian Line (Pthumerian Elder)
Dragonslayer Armour - Amygdala
Twin Princes - Ludwig The Accursed/ Holy Blade
Oceiros the Consumed King - Lullaby for Mergo
Champion Gundyr - Father Gascoigne
Ancient Wyvern - Micolash
Nameless King - Darkbeast
Soul of Cinder - Gerhman, the First Hunter
Champion's Gravetender - Moon Presence
Sister Friede - Lady Maria of the Astral Clocktower
Demon Prince - Laurence, the First Vicar
Halflight, Spear of the Church - Queen of the Vilebloods
Darkeater Midir - Living Failures
Slave Knight Gael - Orphan of Kos

This mod should't have any compatibility issues with other mods, unless they change the game's music too.

Installation:
1- Install Mod Engine: https://www.nexusmods.com/darksouls3/mods/332
2 - Copy the "sound" folder and place it inside your mod directory.

Big thanks to RavagerChris37's and his mod tool: DSSI - Dark Souls Sound Inserter. With this tool this mod was made possible!