When a player during the escape from stratis COOP mission picks up an enemy gun, usually the DIO KH2002 Sama (Katiba 6.5mm) as this is the most common gun to find on Iranian troops; the sound when firing the gun is non existant.
Bullets are there and can kill targets and can be seen hitting the floor, but there is no sound.
Kill the enemies that come over the hill in front of you when you face North at the start of the COOP mission. pick up their gun and fire. your partner in the game may not be able to hear the sound of the gun firing but can see the bullets landing or killing other targets.
Please check the known issues before posting:
http://alpha.arma3.com/known-issues
"Weapons looted from enemy corpses don't emit any sound when shooting in multiplayer"
Thanks