Some people have complained that they can't play YouTube videos -- they start the video and it freezes up after two seconds, and there is no sound. Digging around on the web, I discovered the problem is widespread. Some say it might be due to excessive bandwidth use, others say it might be a bug that effects Youtube.
It seems to only happen when you're viewing via Mozilla Firefox. I'm not positive about this, but it doesn't seem to happen on Internet Explorer. Make sure your add-ons are enabled. Here's a video to try. Compare.