I think if you use a simple movie editing program like Widows Movie Maker it normally seperates the sound and the video so you can edit it - change the volume etc. I think you may be able to save these sounds seperatly but i don't know. I'll hav a quick look...
[Edit] Ok i don't seem to have Windows Movie Maker anymore
lol. Oh well. most video editing software does it so have a look in whatever you have if you have any. I'm not positive that you can save the sound seperatly though so uno lol.