Try the following:

Update to flash player 11.5.

Try the suggestions at Flash 11.3 doesn't load video in Firefox

If that doesn't work try to reset Firefox Refresh Firefox - reset add-ons and settings

Hi, Thanks for the response. I probably wasn't clear enough, but I have already been on Flash Player 11.5 with no success.

Yes, but I would like you to update to 11.5 again, and try more troubleshooting steps with that most recent version of flash.

Hi - yes, no worries, I did what you said anyway, and re-installed 11.5. There is no difference. There is sound to some videos, but not others.

I've cleared the Flash cache and the FF cache. I am very reluctant to reset, but I'll try it as a last resort if I can figure out a way to backup everything first.

Sound for these two videos does not work. One is old, one is new....

http://www.youtube.com/watch?v=I_NKU5ofJfA http://www.youtube.com/watch?v=dXTRANvdouQ

Sound for these two works: http://www.youtube.com/watch?v=sQGujsHcq6U&feature=related http://www.youtube.com/watch?v=tfrWuiQ4QNc&feature=related

A reset does restore your information, but if you want to backup anyway, read Back up and restore information in Firefox profiles

Hi again! I did something similar to reset; I created a new profile. With this profile, the sound works. I don't understand why, since I disabled all extensions with my regular profile, and still had the problem....

But how do I find out what the problem with my current profile is...?

It is set up just so... I've got about 30 extensions active and some of them have settings. I want it exactly the right way otherwise I might as well use Chrome...

I'm going to bet it was your extensions. Even running with them disabled doesn't revert any settings they may have changed that could have caused this.

Transfer your information from your old profile to your new profile, and then re-install your extensions. That is the most time-efficient way to do this. Recover important data from an old profile

As soon as I copied over the information from the old user folder to the new user folder, the problem returned. I've spent 5 hours on this now. :-( So irritated.

I'll reset Firefox but it's clear from the information that I'll have to spend ages then trying to locate all the extensions, do a fresh install of the extensions one by one. This is what I wanted to avoid doing.

Reset did not work either, The only thing that works seems to be creating a brand new profile, from scratch.

Try to boot the computer in Windows Safe mode with network support (press F8 on the boot screen) as a test to see if that helps.

• http://windows.microsoft.com/en-US/windows7/Advanced-startup-options-including-safe-mode

Try to clear the Flash local storage and settings.

• Windows Control Panel > Flash Player > Storage: Local Storage Settings
  • Remove a specific domain: Local Storage Settings by site
  • Remove all domains: Delete All and Delete Data

Flash Website Storage Settings panel:

• http://www.macromedia.com/support/documentation/en/flashplayer/help/settings_manager07.html

Global Storage Settings Panel:

• http://www.macromedia.com/support/documentation/en/flashplayer/help/settings_manager03.html