When you think about it, it's not that crazy. A person can get an exceptionally good idea of where sound is coming from, something that games just can't replicate. The best a game can do is general direction, but a human can pick out an exact direction, as well as distance, whether the sound was behind something or direct line of sight (sound), etc, based on how it was picked up, such as bouncing off a wall, combined with the sound bouncing off of other walls and comparing the gap between them, etc. I work in a decent-sized, very cluttered computer room. I can pinpoint a sound anywhere in the room, from anywhere else in the room, to within about a foot, regardless of what's between it and me. That's what Joel is doing, the same thing any other human can do. He's listening and pinpointing the location of the enemies in his environment, regardless of whether he can see them.. I can do it, you can do it, anyone can do it. The game is just doing it in the only way it can... visually.