The information submitted with your question indicates that you have out of date plugins with known security and stability issues that should be updated. To see the plugins submitted with your question, click "More system details..." to the right of your original question post. You can also see your plugins from the Firefox menu, Tools > Add-ons > Plugins.
Next Generation Java Plug-in 1.6.0_20 for Mozilla browsers
Check your plugin versions on either of the following links':
Re: I have no sound when I watch video with Firefox but I have sound when I watch with Internet Explorer.
I had the same "no sound" problem with Firefox 13.0.1. Also, I had the latest flash player plugin. I completed removed the flash player plugin. I re-installed older flash player version 10, file name: flashplayer_10_plugin_debug.exe, now the sound is back and perfect.