Such is the world today with google that people who understand that problems exist are apt to simply type into google the important bit of information about their problem and call up a nice page like this to find out why exactly with Firefox and Flash their embedded Youtube videos play for about two seconds without sound before stopping. Even with the download itself finishing and refusing to show you the video. It did it here and there in my previous version of Firefox but upgrading to Firefox 3.0 and clearing out the profile and starting from scratch it should have corrected if it was my problem.
The problem is with Flash 188.8.131.52 and the solution is downgrading:
Uninstall the current version you have installed get a copy of 184.108.40.206 and watch videos again.
The instruction for uninstalling are a bit draconian (get uninstaller, run it in the command line! ZOMG!) but required.
To celebrate... here's Hey There Delilah (are you suppose to italicize music video names?). Google visitors coming in to see if there's a solution to your problem... don't you wish you could play this?