This may have been a temp glitch with the source website.
Can you give us aa link to a topic and we can report back to you if we can also hear sound. As some videos just dont have sound.
It could also be that your youtube play you have the sound on mute on the player and other say News
sites used there own video players and there soud is fine because they have loaded up with the default settings and you hear sound.
From what you have said, i think you have hit the mute on a player and its remembered your settings.