This information might be useful for anyone who has this problem.
Have you found that when you try to play a video in YouTube, you get the sound but not the picture?
If so, this relates to a recent Flash Player update, and there is an easy fix.
1) Go to this page: http://www.youtube.com/swf_test.html
2) In the video that appears, right click inside the video and select "Settings".
3) You should see "Enable Hardware Acceleration".
4) UNTICK the box next to it.
If you don't see this option, there is a row of "tabs" at the bottom. You need to click the tab on the far left that looks like a TV screen.
5) When you have done this, close down your browser, then reopen it. YouTube videos should now be fine.
This problem can affect any browser (Firefox, Chrome, Safari, Opera, IE, all versions).
Attached is a screenshot showing the settings box you need. UNTICK that box!
NB: You must visit the link above in the same browser you are having problems with.