I’ll add one more thing.
The tool that I found indispensable in the past was this:
Total Recorder:
http://www.totalrecorder.com/
The VideoPro version is $53.
I was able to manipulate video and audio with very high quality results.
There’s a trial version you can try first.
Can’t you do the sound separately and add it as a track? Any good or even a bad video editor allows that. Audacity is a pretty good sound editor