See:

• https://support.mozilla.com/kb/No+sound+in+Firefox
• https://support.mozilla.com/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.

Hi \ Thanks for the answer, but i am using XP, so there is no volume mixer yet...

Your above posted system details show outdated plugin(s) with known security and stability risks that you should update.

• http://www.mozilla.com/plugincheck/

1. Next Generation Java Plug-in 1.6.0_24 for Mozilla browsers
2. Shockwave Flash 10.0 r32

Uninstall the current Flash plugin and install the latest version of the Flash plugin.

• https://support.mozilla.com/kb/Managing+the+Flash+plugin
• http://kb.mozillazine.org/Flash

• http://www.adobe.com/software/flash/about/
• http://kb2.adobe.com/cps/191/tn_19166.html Troubleshoot Flash Player installation | Windows

Update the Java plugin to the latest version.

• https://support.mozilla.com/kb/Using+the+Java+plugin+with+Firefox
• http://kb.mozillazine.org/Java
• http://www.oracle.com/technetwork/java/javase/downloads/index.html (Java Platform: Download JRE)