I saw Flash Mute software and used it. It is an old software and after you install it stays in the start menu. I do not like extra load on my system startup and a bit annoying too that is why i stopped using this software.
I searched for an alternative which will be light. And i found a method which is actually for advanced level user. But i like this process and it is working great for me.
The whole procedure is as follows:
1. At first just go to the firefox installed directory, by default it is in "C:\Program Files\Mozilla Firefox".
2. Be sure to close any opened firefox browser window.
3. Then create a notepad file and rename it as "msacm32.dll". That's it test your firefox now.
4. If you want to enable sound again then just rename that file.