So the 5.1 audio in NGS seems to be pretty messed up, about half of the sound comes out from my front 3 channels as expected, but a lot of sounds like footsteps, other people's gunshots, etc come out of my rear two speakers, and very loudly.

Something is up with 5.1 mixing for sure. Most people probably won't notice this because they're on 2.0/2.1/Headphones, but I could tell instantly that something was up with 5.1 in NGS. I don't have this problem in PSO2.