This mod replaces every single song in The Binding of Isaac Rebirth (and the Afterbirth Expansion) with appropriate songs from the Undertale soundtrack by Toby Fox.

Installation is easy- Just take the "music" folder and place it into your game's "resources" folder.
This is located at:
steam/steamapps/common/The Binding of Isaac Rebirth/resources.

You might have to turn the music volume up a tiny bit, because the Undertale songs sound a bit quieter than the original Rebirth songs.

Special thanks to Ellis Freeman on steam for making the tracks loop better.

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FULL LIST OF CHANGES:


FLOOR MUSIC:

Basement -------------> Ruins (Ruins theme)
Cellar -------------> Enemy Approaching (Battle theme)
Burning Basement -----> Another Medium (Hotland theme)

Caves -----------------> Waterfall (Waterfall Theme)
Catacombs ------------> Uwaa!! So Temperate
Flooded Caves ---------> Quiet Water

Depths ----------------> Spear of Justice (Undyne's Theme)
Necropolis ------------> It's Raining Somewhere Else
Dank Depths -----------> CORE (CORE Theme)

The Womb -------------> Here We Are (True Lab Theme)
Utero -----------------> Here We Are (True Lab Theme)
Scarred Womb --------> Here We Are (True Lab Theme)

Blue Womb ------------> Amalgam (True Lab Battle Theme)

Cathedral ------------> Undertale (New Home Theme)
Chest ----------------> Death By Glamour (Mettaton EX Theme)

Sheol ----------------> Battle Against a True Hero (Undyne)
Dark Room ------------> Spider Dance (Muffet's Theme)


SPECIAL ROOM MUSIC:

Angel Room -----------> Fallen Down
Devil Room -----------> You Idiot
Library ---------------> Home (Toriel's House)
Arcade ---------------> sans. (Sans' Theme)
Secret Room ----------> Mysterious Place
Store ----------------> Shop
Boss Room (Cleared)----> Reunited


BATTLE MUSIC

Ambush ---------------> Dummy! (Dummy Battle Theme)
Boss Fight ------------> Bonetrousle (Papyrus' Theme)
Afterbirth Boss Fight --> Metal Crusher (Mettaton's Theme)
Mom --------------------> Heartache (Toriel Battle Theme)
Mom's Heart/It Lives ----> Your Best Nightmare (Flowey Battle)
Isaac -------------------> ASGORE (ASGORE Battle Theme)
??? ---------------------> Hopes and Dreams (Final Battle)
Satan -------------> Song That Might Play When You Fight Sans
The Lamb ---------------> MEGALOVANIA
Ultra Greed -------------> MEGALOVANIA
*Note, several bosses use each other's themes*


MENUS:

Main Menu Theme ------> Once Upon a Time
Credits ---------------> Bring it In, Guys! (Credits Theme)
Game Over Screen------> Determination (Game Over Theme)


MISC SOUND EFFECTS:

Boss Defeated ---------> Good Night (first 3 seconds)
Boss Intro -------------> Oh My... (Mettaton EX Intro)
Floor Transition -------> Memories
Death Sound -----------> Sound clip of Soul Breaking
Loading Screen --------> Sound clip of starting a new game
Discover Secret Room ---> Ending of "Home (Music Box)"


I may have forgotten a few because I replaced so many. Enjoy and let me know if there's anything I missed!