The sound has been turned up, but the n-bomb and its contect are actually more clear on the original.
Sorry, “context”.
And that's all the doctoring that took place, as far as I can tell.
To my knowledge there is no software readily available that could convincingly "doctor" Kantor into appearing to drop the "n" bomb. They would have had to use an actor, and a damn good one, and a first-rate film editor to so doctor this clip. It would cost thousands of dollars to do professionally and you'd have to be very confident that neither the actor nor the editor would ever talk.
Kantor said it, and he got caught saying it. Pennebaker's just in the tank for Clinton.