Minecraft wiki entry for Development_versions, associated text: Added 3 new soundtracks to the Nether, composed by Lena Raine: "Rubedo", "Chrysopoeia" and "So Below."
"Rubedo" plays in the nether wastes biome.
"Chrysopoeia" plays in the crimson forest biome.
"So Below" plays in the soul sand valley and basalt deltas biomes.