As requested, I've adjusted this mod to play 2 more classical pieces for the 2 new stages presented in the Fall from Grace mod, Boiler and Grotto. I am not by any means a coder, so hopefully this should work in lieu:

1. Download the folder "music" (as a ZIP file) from this link here [mega.nz] and unzip it.
2. Go to your mods folder - usually in the path of C:\Program Files (x86)\Steam\steamapps\common\The Binding of Isaac Rebirth\mods\
3. Click into the Fall from Grace mod folder.
4. Click into the "resources" folder.
5. Paste the previously downloaded "music" folder and overwrite the existing one.
6. Ta-da!

Have fun!