Well I dunno if he already figured it out but mc9367 wrote this in a topic:
"i really should be putting this up on the xbox board but it seems rather dead. anyhow, my problem:
basically i'll be playing the game for a couple of hours. after a while, the sounds of the enemy's guns no longer make any noise. all i'll start to see is flashes from their gunfire, the sound of my own gunfire, and the occational voice over. has this ever happened to anyone???? you wouldn't believe how hard the game becomes when you have no idea where you are being shot from!!!
i threw my copy out, and am right now downloading a copy of the game via xbl. it would just be crappy to know that this is a common problem that was never truly addressed, especially after just spending the mone"
Well in case you haven't figured it yet, it's no glitch. I own the ps2 version and evrytime that happens is because you only have about 1 or 2 bars of life. everything gets kinda slow motion and you just hear your heart beat, the gunfires, not much besides that. I really like that effect =]