Flash-based sound does not play correctly
I get sound and video on Youtube and other Internet video programs through Microsoft Explorer. But on Firefox i get video with no sound. Have uninstalled and reinstalled Firefox twice. Have uninstalled and reinstalled Flash plugin twice (it's the Firefox plugin.) Have gone through all troubleshooting pages and suggestions on Adobe and Mozilla Web sites, such as the "managing the Flash plugin" tutorial. Uninstalled and reinstalled Quicktime. The one thing I haven't done is to check "Profiles" because I can't get that to load.
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See What to do if Firefox won't play any sounds and Fix common audio and video issues
You can click the Volume icon on the Windows Taskbar near the clock and click the Mixer link at the bottom to open the mixer.
You can adjust the volume level for each application in that window, so make sure that Firefox is not muted or set at a low level.
You will only see the Firefox program in the Mixer if you are on a website that plays sound.
You can also try to clear the Flash cookies and settings.
Flash Website Storage Settings panel:
See also Flash Local storage settings: