It would be simple for him to loop new audio for his parts after the tape is recorded. Edit out any exculpatory parts and you've got something that sounds bad. You're right, it is about context. I don't recall her mentioning prostitution. He is the one who talks about hookers and underage prostitutes.
Well it's not as cut and dried as simply editing it in. He'd have to make sure he wasn't talking at any point where she was talking and there wasn't any other visible sources of sound such as her flipping through papers where they would want to make edits. Splitting the sound out from two different sources on the same track is hard. You can enhance and mute areas but removal is not an easy task. As far as the level of his voice since the camera was closest to him and/or he was probably wearing a mic that would explain it. Wireless mics aren't that expensive. I have one myself I used for my Cousins wedding.