That particular link leads to a live news commentary (I presume, I don't speak Arabic), and at the top of the Quicktime Player window it says 'Audio'.
But having tried to play some of the videos listed on the page, I do only get audio. Quicktime Player's Movie Inspector shows N/A against the video track so we cannot tell what codec it is, although I would have thought it was Flash videos.
I can only suggest that you contact Aljazeera about the problem.
Note however, that the videos you can link to from the English home page:
http://english.aljazeera.net/
play just fine.