Could be because of incorrect Flash Player settings or Sound Files corruption or even due to conflict with the add-on installed.
Suggestion:
1.Check the Sound settings on Flash Player by right click on the Video
2.Open Internet Explorer -> Tools -> Internet Options -> Advanced -> Multimedia -> Place a check on Play sound in web pages
3.Uninstall the flash player by going to Start -> Control Panel ->Add or Remove programs (If Vista then Programs and Features) -> Double click on Flash Player to uninstall