Simply put infected sounds are correctly played from the source direction (left, right, behind, in-front). However the direction does not change once the cue has begun playing.
e.g. a zombie directly on your left, will produce a sound through your left headphone but If you change direction (camera or player)(during playback), so that the infected is now on your right, the direction of the audio will not change and will continue to play through the left headphone.