Try this

• https://support.mozilla.org/en-US/kb/fix-common-audio-and-video-issues

Hello,

Try Firefox Safe Mode to see if the problem goes away. Safe Mode is a troubleshooting mode, which disables most add-ons.

(If you're not using it, switch to the Default theme.)

• On Windows you can open Firefox 4.0+ in Safe Mode by holding the Shift key when you open the Firefox desktop or Start menu shortcut.
• On Mac you can open Firefox 4.0+ in Safe Mode by holding the option key while starting Firefox.
• On Linux you can open Firefox 4.0+ in Safe Mode by quitting Firefox and then going to your Terminal and running: firefox -safe-mode (you may need to specify the Firefox installation path e.g. /usr/lib/firefox)
• Or open the Help menu and click on the Restart with Add-ons Disabled... menu item while Firefox is running.

Once you get the pop-up, just select "'Start in Safe Mode"

If the issue is not present in Firefox Safe Mode, your problem is probably caused by an extension, and you need to figure out which one. Please follow the Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems article for that.

To exit the Firefox Safe Mode, just close Firefox and wait a few seconds before opening Firefox for normal use again.

When you figure out what's causing your issues, please let us know. It might help other users who have the same problem.

Thank you.

Thanks - I did review the info at this link before posting - nothing helpful there.

I've had this problem quite a few times and it mainly happens after I use Mumble (it lasts for a few days if I stop using Mumble for some time). What I would do is click on the volume icon on the tool bar which will show the volume level, I'd then click on Mixer and see if all of the audio levels are the same.

My issue is more with Adobe Flash Player 11.9 r900, but doing this, which is a pain in the butt, has always fixed it. There have been times where I will have to do it multiple times a day, but it works. I hope this helps.