Sometimes when I am playing a mission my sound is almost "muted" when my camera is directly behind me. Whether it is in a vehicle or on foot. When I turn my camera to the left or right (ALT Look) the sound is back at its normal state. I would have to say it is about 10 degrees on each side off of the center view when the camera is facing infront of the character. It gets quite annoying when I am trying to play a tactical mission in the game, where silence and strategy is the key.