I recently downloaded a music video using Shareaza as well as another video file and both downloaded like the old Raid pesticide commercial with image but no sound!

In fact most of the videos I was attempting to download at the time had no sound when I previewed them and at first being a novice on this program thought that the previews had no sound but then just a single one of the many videos I had been downloading previewed with sound. So my question is why are most videos on Shareaza apparently without sound?