I can't seem to get it to work. :o/ The video track is in video (under Video Overlay) and then the audio track is underneath that. I want to mute the video clip to add different files to make a vid. :o/
Do what Birk advised, except delete the video instead of the audio. However, this may not even be necessary. Anything you place on video tracks above the video will cover what's below it. You can probably leave the original video there and ignore it.
Another option, assuming the unwanted video is the only thing on it's track, is to click the mute button in the track header. This will prevent anything in that track from being seen. You could also bring the composite slider in the track header down to 0%. If you need to mute some events on the track and not others you can hover the mouse pointer at the top edge of the video event and drag down. A thin blue line will appear and the lower you drag it, the less of that event will show through. Drag it all the way to the bottom and that event "disappears".
There's lots of different ways to accomplish this, but simply putting wanted stuff on a track above the unwanted video is probably the easiest.