You may have better luck engaging the services of a lip reader to go through the videos.

We actually get requests like this every time, and in the vast majority of cases, if you cannot hear them, they're probably not recoverable.

The other thing, being very frank, is that the recovery of the audio is NOT the issue. Trust has broken down, you already know he's guilty of whatever you believe he has done, so any audio recovered will be just confirmation. Worse, the reality could be that a partially recovered fragment could damn him when the rest of the conversation would have proven his innocence. You assume the worst, and what is the point? As you say - it is eating you up. He's a fella and I cannot remember anything I said in conversations yesterday. Often, my wife recalls things I have said that I have no recollection of. Fellas in general don't memorialise what we say. We talk rubbish most of the time.

If he says he cannot remember, you clearly don't believe him. This won't put your mind at ease. I believe you have two options. You believe what he tells you and move on taking his assertion of innocence, or you flat out think he is lying. The outcome is one of two choices. From his perspective, you do not believe him. If trust has gone so you cannot trust him, what will you do if you heard the words? Just delay the inevitable? This time innocent, so you get him next time?

You need to get together and decide if you trust each other or not. If you do, your relationship is safe and solid. If you simply are convinced but want proof - it's gone, finished, done. Start the paperwork.

Me personally - if a client came to me with this as a paying job, I would probably turn it down. I don't wish to get involved in something so terminal for one of the parties, even with payment. Sort your relationship between you, and forget this audio silliness. whatever we find, you will still not believe him.

this sounds like a job for a private investigator, not an audio engineer.