Customs songs from mash-ups never play when playing in certain biomes, only default ones.

How to reproduce.

• Go to 'settings > resource packs' and load any Mash-Up with custom background songs (be sure is at the top of loaded resources).
• Start any map on survival.
• Play for a short or long period in a forest biome, a taiga biome or even in the nether, the Mash-Up's background songs won't play.
• Go to a plains biome, a mesa biome or even to the End, quit and reload the map. The Mash-Up's backgroud songs should start playing.

The next table shows the biomes that where used to test this issue (there may be more biomes where this issue is happening or not).

The next table shows all three locations where this issue was tested.

Mash-Up used:

1. Master Chief Mash-Up