Why can't there be sound in browsers?

Question

Browsers

Win XP operating system. The sound disappeared, and selectively - on some videos there is, on some there is not.

For example, in this topic about Felix there is sound, but if you watch from YouTube, then there is no sound.

Flash player latest, just installed. Latest codecs, K-Lite Codec Pack Mega
I mainly work with opera, version 12.02, build 1578, but the problem is observed in all browsers.

Suggest solutions.

UPD. I also noticed that if there is a “Full Screen” button in the embedded video from YouTube, then there will be no sound. If there is no button, then there will be.

Answer 1

Maybe in some places the videos are in HTML5, and in others - in Flash?
Check www.youtube.com/html5

Answer 2

I also think that the problem is in Flash. I myself constantly fight with him after the release of a new version. Try the opposite of an older version, such as 11.2, to localize the problem. Google says that sound problems have been going on since version 11.3.
Archived Flash Player versions