I, too, am having the same issue. Only since Windows 10 upgrade. Only on Opera, not Edge, not Firefox, not Chrome. I've reported it to Opera twice and haven't heard anything back. It's still broken as of today 09/13/2015. I've tried the fixes listed here and they don't work. Happens on YouTube as well as embedded videos. I like the speed dial in Opera but it's annoying when an embedded video starts to play and the audio is so choppy. I have to go to another browser.
Opera About = "Version: 31.0.1889.174 - Opera is up to date Update stream: Stable System: Windows 10 64-bit (WoW64)"
Like 'freezeware' above, I'll stay tuned until someone finds a fix.