Might be an incompatibility with these three mods:
Amplified Nether (Makes the nether 256 blocks tall)
Better Nether (Adds new nether biomes and blocks)
Immersive Portals (Allows you to see through portals)
Sometimes I can hear the nether sounds if I don't stay in it for too long. Most of the time though the sound just cuts off as soon as I enter. Once it's cut off in the nether, it will affect all other sounds in the game, including menus. That's about all the info I can give, as I stopped using this mod after I found the glitch.

I believe this is an issue with Immersive portals affecting how the game manages changing from the nether to the end. This will be worked on but is not a top priority.