See Fix common audio and video issues and What to do if Firefox won't play any sounds for some solutions.

If you need more help, please post a link to a couple of webpages where you do not get sound so someone can check it out;

I mainly view: youtube, rediff.com, indiatimes.com, yahoo.com etc, there are so many where I am not listening any sound, music or speech, while browsing these sites in firefox.7.

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.