When I would put files on my site, there were often comments that some members couldn't view or listen to them. I've seen similar comments for other sites such as Megaupload. I myself recently had problems getting the audio part of videos on certain sites. I found this free codec pack on CNET (to me one the top and most trusted sites on the net) and now I have no problems with any kind of media in any format. When you install it, you may get the Dealio tool bar which can be uninstalled.
http://www.download.com/Media-Player-Codec...4-10749065.htmlJust install it and your media problems (if you use Windows Media Player of course :) ) should be solved.