clone mode

That is because of the way video is decoded on your graphics card using an overlay.  That is the same reason why you cannot use the PrintScreen button to take a snap-shot of a video.

You can try changing the Media Player setting to not use an overlay (see below).  This will cause all the work to be done on your main processor instead of the graphics card, might cause frame rate to drop.  You may be better off not using clone mode and setting the TV to be your only display when you want to watch videos (if you have a laptop, you can quickly toggle with the FN+F8 button).

Open Windows Media player and pull down the Tools menu and go to options.  On the Performance tab press Advanced.  Uncheck the use overlays options (note, there are two, one for video and one for DVD).

Could it be a feedback loop?  (computer's output connected to receiver’s input and receiver’s output connected to computer's input?)  Open your computer's mixer settings and mute each item one at a time.  If you still hear the hum with everything muted then you might try connecting a different source to the same receiver input (such as a portable CD player).  If you don't get the hum then it is probably the computer's sound card (consider buying a high end one with a digital output).