What about other sites... Try www.Hulu.com
in firefox then IE. They are one of many sites springing up offering high quality or HD full length tv episodes and movies for free with just 4 or 5 15 sec commericals.
Pick an episode or movie make sure the video window volume is up and let me know if the volume is the same or normal.
I have to admit, you have me stumped if that does not work. You might try reinstalling or updating your audio driver and or check if you have more than one audio driver installed on your system.