Did you already try to disable Flash hardware accelleration? That's kind of a panacea for Flash related problems.
Tried all the the obvious stuff like checking the volume and making sure that the HDMI output was my default sound player.
Not the TV or cable as it works fine on my xbox, plus when I go into the HDMI output properties and use the sound test it plays through the TV, just wont play stuff like youtube videos etc. Any help would be appreciated guys thanks!