Check for any other software that is running and set to 'mute'. The slider only displays the status of the volume mixer device, and the device itself is shared between all the programs - if any of them changes the volume, others should move their sliders accordingly. If one program keeps muting the volume, setting it in another won't help.

see also

What to do if Firefox won't play any sounds

Is this happening on the YouTube website?

YouTube is aware of this issue if this is the case:

• http://productforums.google.com/forum/#!topic/youtube/ETfCgHO4MhU

You will probably notice a mute X next to the (speaker) volume icon on the control bar of the Flash player on the YouTube site (hover the player).
There seems to be a problem on the YouTube website and the sound is muted by default.
So you need to hover the speaker icon and move the volume slider to the right to un-mute the sound.

I ve tried that but the problem is not only in youtube.It is in everything that uses adobe flash player in firefox.

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

I was able to fix mine by decreasing the sound for just flash in the mixer. That seems to have knocked the mixer out of whatever issue it had with flash. Before that I tried most of the advice above. I also tried muting all currently running programs and then unmuting them one by one. I restarted my computer to make sure the issue was resolved and it is. Hope this helps someone out there.