Do they ever use the actual sounds/voices that are captured during filming or is it always added in afterwards? Also, how often do you think the voices ARE NOT even the performers themselves?

If Wankus has a heart attack and yells out for help but nobody responds, did he make a noise?

Porn rarely uses voiceover. You have to think of low production costs and the time and talent involved in doing voiceover and foley.

Depends on what you mean by voice overs.... Often narration is done over the action in a movie rather then having the porn stars do dialog. Sometimes after a scene is filmed you find the voice or sound quality came out badly and you have someone redo the moans or the dialog, (or just cover it with music in post, lol.) I, personally, really like doing voice overs or narration and have done some for porn and for mainstream. If anyone needs some done let me know!
Cindi Writercindi@aol.com

I remember watching one of those cheap, really crappy, 4-hour tapes(you know, the ones that usually cost less than $20 at the adult stores and don't have any of the scenes pictured on the box in it) and during one of the scenes the grunts and moans did not match the performers mouth movement(like, the woman's was mouth wasn't moving, but a female voice was saying "oh, god" "fuck me" and whatnot). The voice track also seemed to be repeating itself every 2 minutes.
I've also noticed some voice-overs in some Max Hardcore movies(US versions). Sometimes he'll ask the young-looking harlot how old she is, and she'll reply "I'm eighteen" even though her mouth looks more like she's saying fourteen or thirteen. Kinda creepy.

So you guys are saying that what you usually hear on the video was actually being heard on the set at that time?

I remember when I was working at VCX and going through all the classic footage they had. There were a few that had music right off of the radio, commercials and all. As for the audio matching, sounds like it got out of sync. in post or there were a lot of audio interuptions (directors love to hear themselves speak)and the guy was just using audio cut from a clean part of the raw footage.