No matter how long you stay in a Basalt Delta, its biome-specific song "So Below" does not play. It is not a problem with the track itself, as I have tested it in a Soulsand Valley and it plays as it should. This was also an issue in the 1.16 betas that was resolved but has occurred again in the latest release. I have run multiple tests and received the same results; in Basalt Deltas it only plays the basic nether songs ("Concrete Halls", "Ballad of the Cats", "Warmth", "Dead Voxel"), and when I run back to a Soulsand Valley, "So Below" plays.