Which player do you normally use to play the media files? I would normally recommend reinstalling the player - e.g. MediaPlayer, WinAMP, etc, which will then sort out the file types.
Another method could be to download the media file (i.e. File/Save As) rather than playing them directly off the site, and then see if this is the problem.
Anyway, let me know, and (hopefully) see ya at Glasgow!