In any normal/regular server, playing with friends or solo, sounds of cans opening, ammo being split and zombie aggro are triggered without any of the actions being performed by the player; just the initial attack of the sound, not the whole sound clip/file, just the initial attack. This doesn't happen in every game and sometimes multiple instances of the same clip/file is played back to back. The exact same sounds are heard by friends in the same server at the exact same time.