Sound problem after upgrade to Windows rt 8.1

[DorsetLyn]

I recently failed to stop my Ativ Tab upgrading to Windows 8.1 (presumably the rt version). I now find that, although the sound works (e.g. I can listen to the radio on it) I cannot get any sound on Youtube, though I can see the video. I've ensured the latest Windows updates are installed. Yes, the sound is turned up!