I'll take a wild guess. A problem with the plug-ins that run the movies in your browser. I'm not an expert here, but it seems several 3rd party video/audio plugins are often called upon, as well as Quicktime. However, control of the plugins is not made easy, and I don't know details. I would check the Preferences of the browsers such as Safari/Preferences/Security to make sure Java and Plugins are checked. I'm guessing a reinstall of some media players like Flipformac might be needed - I think they install plugins for the browsers.