I had this problem (like so many others it seems) that when i played a movie file from my hd through the tv-out on my graphics card (ati radion 7000) all i got was audio and a blank screen. well eventually i found a solution.
right click on desktop
left click on properties
left click on settings
left click on advanced
left click on displays
select tv as primary display and monitor as clone
you will only see the movie on the tv (monitor will be blank)
its a bit of a pain but at least it gets around the problem
hope this helps somebody