For sure that you have downloaded an avi video which seems to be without sound. It’s usual that they seem to be like that, but the real reason is that they include the sound codified using AC3 compression and you don’t have the correct codec installed.
This audio compression format is compatible with DolbySurround, Dolby ProLogic and Dolby ProLogic II and is more and more used. If you want to enjoy your videos with their sounds, install this codec and put no limits to your sound experience.
This DirectShow filter for AC3 is totally free and distributed under GNU license.