Mashup pack music is not working, normal music plays instead, except for on the main menu when the mashup pack is applied to global resources. Tested with the Festive Mashup 2016, will include more details as an edit as I do more testing.

Edit: Festive Mashup survival music seems to only work when played using the playsound command based on testing up to this point

Edit: It's working as expected in some worlds, but not others, not sure why.

I'm currently doing testing to figure that out how to reproduce the issue and what packs it affects.

Edit: Currently it seems as if survival worlds created with the resource pack active are having the issue, will test if disabling and reloading the world before reactivating the pack fixes the issue for those worlds.
Edit: Did not fix the issue, will test with a creative world.

Edit: Seems to work as expected in a creative world created with the pack active, will now test with other mashup packs. If someone comments with more packs they have tested, I will add them to this list

Edit: Same results with Master Chief Mashup Pack

Edit: Nightmare Before Christmas Mashup seems to work as expected

Edit: Halloween Mashup seems to work as expected