This could be a codec issue, download the KaazaMega Codec Pack, then reboot, this should cure the issue. think of the movie file as a card device in your computer, this card needs a driver for the computer to know how to use it, in much the same way the computer also needs to know the codec that a movie file was created in, and if you dont have the correct codec , then the movie will not play correctly. the mega codec pack has many of the more common codecs used today and should have the one that you require for your movies. I dont know of a website that has the pack, as I soursed mine off of emule p2p client. but I am sure you'll find it somewhere.
Here's one for you inquisitor. I download videos, can see the images but have no sound. No problem with the speakers, theres just no sound for video's, music, anything. Just seemed to happen all of a sudden. Virus maybe?
gardna, first play the movie on another PC / PSP or something to make sure there is sound encoded in the file in the firstplace.
P_E_E, download it from here : http://codecpack.nl/klcodec272f.exe
Gardna: Not all video found on the web have sound, but have you checked all your wiring and plugs between your computer and your speakers?. Maybe, the wires are not plugged in the right port?
P_E_E: Inquisitor_UK have already answered your question.