I played once with crappy headphones that made every sound sound like if it was just next to you (lucky thing not all enemies spawn at once) I was like 30 meters away from some grunts that couldn't see me, and suddenly one of them shouts "we're all gonna die!" without any reason at all. After a second or two, another (or same) grunt said "Cover me" a third (or not) grunt replied "Roger that!" Then I waited for ten seconds, and suddenly a grunt jumped side wise out from cover. I shot it in the belly, and it said "he's everywhere!". Then all the other grunts started to scream and sob, and with my headphones, that was really painful. And I was still like 20 meters away.