Actually they aren't too bad, provided you turn the sound down a little and keep your hand away from the pause button. Pause is not your friend here.
It's hard to tell who at developer Rebellion is having more fun making this game - the animation guys, figuring out creative new ways to mangle 3D models, or the sound people, working on the screams, hisses, and juicier bits involved with the kill moves in Aliens vs. Predator. I suppose it's entirely possible that they're simply recycling old sounds, but I'd like to imagine that somewhere right now a man is taking out his frustration on a watermelon with the pointy-end of a claw hammer, surrounded by microphones.