I have the same problem sometime About 10% of all video's i record have low volume. Even if you hook computer speakers to your computer the volume is still so low you can not hear all the words.

So i tried DFX Audio Enhancement it works great on the volume,you can down load the trial version for 30days you only get half the volume but you will be able to tell the difference. fxsound.com

They have a package that covers just about anything you will ever play on your computer or you can get it just for windows media player,Div x player or what ever.