While the old nether music tracks by C418 can play generically in-game while the player is in the nether, they aren't actually possible to hear via the /playsound command. Playing music via the available nether sound events will always result in the biome-specific tracks by Lena Raine being played.

For example, using commands to play nether wastes music (/playsound minecraft:music.nether.nether_wastes music @s ~ ~ ~) always plays "Rubedo". If it's intentional that these events only play the biome-specific tracks, then there should be a generic _music.nether sound event added for the other tracks.