About 10% of the YouTube videos I watch have no sound, mostly when I click on a link that takes me to a video. Happens on some music videos & most Major league baseball vids. HELP!!
Maybe you have some browser addons conflicting with it. Have you tried using a different browser? Have you checked the volume bar inside youtube window? Sometimes I have seen where it has been muted.
If you didn't upgrade the browser, then you can also try delete the video/audio related plugin, clean history/cookies/caches to see if the problem still exists.