Could be a browser issue, possible trying a different browser may, but I did find a Troubleshooter though below...
If you don't hear sound when playing a YouTube video, try these troubleshooting steps:
1. Adjust the volume on your computer and speakers.
2. Adjust the volume control located in the lower left corner of the YouTube video player.
3. Restart your browser.
4. 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:
1. Update your Flash Player to the latest version.
2. Allow third-party Flash content on your computer. Learn more by visiting Adobe’s help page.
3. Check any Antivirus and Firewall software to see if its blocking third-party Flash content.
_________________
"When you begin to realize your own existence and break out of the social norm, then others know you have completely lost your mind." -PerfectlyDarkTails
AS 168/200, NT: 20/ 200, AQ=45 EQ=15, SQ=78, IQ=135