Once or twice you must have downloaded a video in AVI format that seems like it has no sound. It is possible that the problem is the sound is codified in AC3 format, and you don`t have the right codec.AC3 is a audio codifying format that`s compatible with DolbySurround, Dolby ProLogic and Dolby ProLogic II which are used more and more. To enjoy it, all you have to do is install this small codec: AC3Filter.AC3Filter is a DirectShow filter for AC3, totallly free and distributed under the GNU licence.