Andy,
You probably have a little glitch or disturbance in your video. Oftentimes, when this happens, for some reason, you’ll get an image but audio will not play over the firewire at that spot. You’ll get audio if you export to a DVD, but not out to tape. Take another look at your video at that point, and I’ll bet that you’ll find a little disturbance in the picture. Go back and look at your source footage, and if you don’t see it there, recapture. If not, see if you can find a cutaway to replace that piece or eliminate it all together. It’s probably only a few frames of video.