I can hear sound from any media players, but I can't hear sound from online video streaming websites like youtube, what happened?
try uninstall and reinstall flash player. see if it helps. good luck.
Are you are using Firefox and opening many youtube pages in multiple tabs?
hmm... try another player... somethings, the media player cannot decode the video totally... try reinstalling codec.
I don't know what happened, but it got back to normal after I restart the computer.