there's a good way to find out if someone is using sound or wallhack, go to where they were camping (sneak) and fire a bazooka close to where you think they are (loud explosion). If the player is rather stunned afterwards, they had their headphones all the way up evil:

personally I hate using headphones, I know it's a good advantage but I can't take it, too much fucking noise, especially those damn STGs. Anyway I listen to Hendrix when I play, it puts me in "the zone".