sounds like a codec problem.
Lets start here: Right click my computer, select properties, click hardware tab, and then click device manager. Within device manager, click the plus + sign next to "Sound, video, and game controllers," double click audio codecs, then select properties tab. You'll see the codecs installed on your system. Starting from the top, double click each codec and you'll get a dialog box that allows you to disable the codec. Disable it and test your sounds or video. If that dont solve the problem, go back to the codec, enable it and move to the next one. Once you have found the culprit, write down its name, then try the video/sound. Your sound/video may work fine, but if your media player says it cant find the right codec, google it.