I am using Windows 7 Home Premium with 64 bits. When I view news clips on, say, cnn.com I can only hear the advertisement at the beginning of the clip. However, youtube videos work fine. This is true for IE9, Google Chrome and Firefox. I do have the latest version of Adobe Flash Player installed. I have also lost sound on Skype.
Thanks for the heads up on the bug report. I took a quick look at the dxdiag and it looks like you video drivers are out of date. Could you update them to the latest version?
Someone on the MicrosoftAnswers forum had a very similar problem and fixe it by following the procedure in http://www.sevenforums.com/tutorials/3413-repair-install.html