I suspect that this happens when a player goes unconscious, but I am not sure.
Everytime I play for a while, I hear a random voice speaking. So I look around, but there are no players around me. And the voice is very clear, it's like as if they're speaking in global channel, or similar broadcasting channels (like group, side, etc.), which is extremely annoying.
The way to solve this is to switch server.
I've tried waiting out to see if the voice disappears, and in some instances it does disappear after a while, but sometimes it just stays for a much longer time.