The soundcard thing is probably a reference to sound whoring. You use a surround headset or CMSS3D to get good spatial audio and then alter your EQ to better isolate footsteps. Making it much harder for people to sneak up on you and giving you the drop on player locations.
I've seen those guys that just run around at full speed, OHK'in people with a shotgun then energy transfer and off again. Mobility and Stealth Enhance. Farming kills on objective maps. Blah blah, a 3+ KDR is reasonable. I find it pretty boring to play that way but some people are all about KDR.
I play all the time with people who seem suspect and I think that if they could just alter various stats - like armour level, speed, damage etc - it would be almost impossible to detect it. I don't know if I'm just being paranoid but after playing awhile you do get a feel for what seems right and wrong.
Crysis 2 is fun but it's basically deathmatch. Battlefield is where it's at for real good play.