Adding Google Chrome to the recent 64-bit release, I found that sound on YouTube stuttered. I searched the net and found the solution:
1 - Run Google Chrome
2 - Enter in the address bar: chrome://plugins/
3 - Near the top right of the window, click on the + sign next to the word Details
4 - Scroll down until you find an entry: "Adobe Flash Player"
5 - In that section you will see the word "Disable" with a link three times. Click on the top one.
It seems that it's the Google Chrome flash player that's causing the problem and disabling it allows the other one, the mozilla plug-in, to be used. It worked for me. I'm not sure that this is necessary on all hardware - but it certainly was on mine!