No Sound On Video Firefox Not Playing

The Greasemonkey extension with the IE Media Mimic script will make embedded audio and video play. Video or audio does not play (Firefox Support) No. Firefox doesn't play audio. That 'for Firefox' the volume was all the way down and this is why I could not hear anything while watching a YouTube video in Firefox.

Hi, using Firefox 25.0.1 on Windows XP SP3 and sound is not working in Firefox browser. Sound is playing fine in Windows, just not in the browser. I have checked on YouTube and no sound, but video works fine. Also checked other sites with Flash and also just video and no sound. On YouTube I have also changed default video player from Flash to HTML5 and video also working, but no sound. I have double checked that sound icon in flash/html5 is not muted, but instead set at 100%. I also uninstalled the Flash player and installed it back.

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I reseted Firefox from Help Troubleshooting Information Reset. Any idea what else I should check?

One more strange think. I have downloaded Google Chrome and also video working fine and no sound. Also the same using HTML5 on youtube, video working, sound not.