I've heard voices at times when I was extremely tired. For me, It's like an inner voice is saying things without my permission. The most random things will come up during that time too - Things that are furthest from my regular thinking. Other times It's like I'm listening to two people talk through a wall. I know they're talking, I recognize the voices but what they're saying just isnt clear. I can hear them laugh, and cough.. and I just set there and try to focus on what the voices are saying. I usually snap out of it before i actually hear anything specific though.. probably because I focused on hearing them too much and woke my brain up
