Only vanilla songs are playing in the Nether, and only the Nether. Overworld and End both work just fine.

Using:
Minecraft (1.20.1)
Forge (47.3.1)
Cupboard Utilities (1.20.1-2.7)
Biomemusic Mod (1.20.1-2.5)

The file path for the music included was:
.minecraft/resourcepacks/packname/assets/biomemusic/sounds/music/nether/nether#.ogg

Again, the other dimensions worked fine, so im pretty sure i have the right path setup, just thought id include it to be sure.

Also having this issue

this worked for me, note that vanilla music has a higher chance esp when the biome has a music set to play