K. I would suggest updating your drivers, also be sure to check the windows update, it may have updates to fix the problem or new optional drivers for your ac 97 sound card. Have you been able to listen to sound on youtube before or is this a longtime problem? Does sound work on other sites? Have you tried firefox?
here is a troubleshooting guide from youtube.com:
Why can't I hear the audio when I'm watching a video? To make sure that you hear sound while watching videos, please check the volume controls on your computer. Also, be sure to increase the volume on other conventional video players on your computer, such as Real Player or Windows Media. This may have an effect on your computer's master volume levels. Next, check to see if your system meets these requirements for running the Video Viewer: