Adjust the volume on your computer and speakers. Adjust the volume control located in the lower left corner of the YouTube video player. Restart your browser. Try turning up the volume for other video players (like Quicktime, Real Player, or Windows Media player).
If you still have no sound after completing the steps above, these additional tips may help:
Update your Flash Player to the latest version.
Allow third-party Flash content on your computer. Learn more by visiting Adobe’s help page.
Check any Antivirus and Firewall software to see if its blocking third-party Flash content.