While playing with a group we had instances where noises in the environment or player created noises would only be heard by 1 person.
The first occurred while searching for a cow. Only one of us could hear the mooing from a distance away and I couldn't hear them until I shot a cow and scared the others away.
The second was while searching a city (Chernogorsk). I could very clearly hear gunshots from another player (most likely his BK-133) a few blocks away. The other person I was with could not hear a single shot fired from the other player, even when we were within 20' of the gunhots. All of my shots were able to be heard by both of us.