There are a ton of people who have had the same issues. Google is useful. I have fixed the sound problem but the game does like to shut down on its own once every half an hour. I use a gaming PC and other games crash once in a month. Hexxit likes to crash for no reason.
The other problem which annoys me is missing textures. Spiders, creepers and zombies become floating limbs, heads and legs. In NEI missing pictures for some vanilla items and some mods. I have found a correction and have had someone sent me the files needed.
The infernal mod likes to lag my game. Not nice. I die const