I think this is a hell of a box set, and I'm going through them little by little. Interestingly, I played both the 2.0 and 5.1 mixes on Jason X, and there's no audible sounds of the girls getting beaten in their sleeping bags. I swear there were sounds they made...and now its just the thuds of the bags hitting each other. I just looked up the same scene on yourube and their does indeed appear to be noises here that are absent from the bluray.
**Edit** It appears I was right...