See:

• https://support.mozilla.org/kb/No+sound+in+Firefox
• https://support.mozilla.org/kb/Video+or+audio+does+not+play

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. Current Firefox versions use the plugin-container process to run plugins, so you need to check the volume setting for the plugin-container process. You will only see the Firefox program or the Plugin Container in the Mixer if you are on a website that plays sound.

You can also try to clear the Flash cookies from YouTube

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

Flash Local storage settings Help:

• http://www.macromedia.com/support/documentation/en/flashplayer/help/help02.html

I had a similar problem. I was unable for a long time to get sound out of YouTube and would revert to Explorer 9 to get sound along with video. Thanks to this forum (huge thanks) I took another shot, and found this tip. I went to the mixer link on the volume icon, found that mute was on for Firefox and Thunderbird. Turned off mute and upped the volume slider, and tried again to play the Up and Over YouTube. Still no sound.

Tried another approach that finally somehow worked that I did not see above: Firefox Tools>Options>Applications brought me to a list of settings including audio options. I reset most of them to "Use Windows Media Player (default)", and accepted the changes. On came the sound. If the other suggestions from cor-el do not work, you may try this one.

Sorry, but the problem wasn´t solved, any other suggestion besides formatting the pc ?

Clear the Flash cookies (LSO) and settings.

• Windows Control Panel > Flash Player, click 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

Flash Local storage settings Help:

• http://www.macromedia.com/support/documentation/en/flashplayer/help/help02.html

Thank you for your support but I solved the problem on my own, I uninstaled the sound board, restarted the pc and now is working. What I couldn't understand is why it only affected firefox. Thanks anyway for your help.

Okay, I had this problem too so a friend of mine and I troubleshooted for about an hour and we finally found the solution! Whatever you do, do not uninstall your sound card. That will just give you more problems.

The problem turned out to be the latest version of Flash! Once I rolled back to a previous version, I got sound and no problems.  :D For anyone else who still needs help on this issue, here's the link:

Adobe Flash Player 11 Beta 32-bit - CNET Download.com

OK I had the same problem after I had problems with my on board sound card. Bought Sound Blaster card, everything else worked once I set the Sound Blaster card as the default device (as the old one still was present and worked on the front of my case plug ins). Firefox still didn't even after reboots, I updated the flash player still no joy, until updating Internet Explorer, then restarted. Could of been just the restart I suspect but something else possibly sorted it out. I suspect that if I had cleared my flash cache etc as suggested above it may have solved it.