Create ~mods folder into <Install path>\BloodstainedRotN\Content\Paks

Install:
Extract content of Bloodstained_SotN.zip into <Install path>\BloodstainedRotN\Content\Paks\~mods

Uninstall:
Delete content of Bloodstained_SotN.zip from <Install path>\BloodstainedRotN\Content\Paks\~mods

You can also use mod manager:

https://www.nexusmods.com/bloodstainedritualofthenight/mods/56



This mod replaces following area themes in Bloodstained with themes from Castlevania: SotN

Entrance - Dracula's Castle 
Underground/Secret Sorcery Lab - Abandoned Pit 
Inferno Cave - Lost Painting 
Forbidden Underground Waterway - Crystal Teardrops
Dian Cécht Cathedral - Requiem for the Gods 
Towers of Twin Dragons - Tower of Mist 
Livre Ex Machina - Dance of Pales 
Hall of Termination - Dance of Gold 
Hidden Desert - Wandering Ghosts 
Garden of Silence - Marble Gallery 

I tried to keep each area theme as thematically fitting as possible. Each track has a working intro and loop section.


Update: 

There is a bug in Garden of Silence track where:

-The fairy familiar does not sing in Japanese language setting when playing the piano
-The non vocal version of the fairy track does not play when playing the piano

This is due to Garden of Silence file having 4 separate tracks: two fairy songs in English and Japanese, non vocal track (Miriam plays the piano) and the stage theme. I've tried swapping the fairy tracks but then it breaks the stage theme. I'll see what i can do but if this is hard coded, then there's not much to do. You can always leave the Garden of Silence track as is.