with no certainty of success.

I know nothing about this so when I searched Google with the term " How to increase volume on a video recording " I found a few hits, and this one seemed interesting;
http://forum.my.nero.com/index.php?showtopic=7215

But it does seem technical and I don't think the poster there found a complete answer.

Good luck.

Have you thought about sub-titles? That's a UK term for what I believe US people call CC or Closed Captions.

Mark