[jjd-uk]

The graphics(&audio) driver should be the newest one from Intel:

Have you tried with the drivers dated 02/20/2014 ?

https://downloadcenter.intel.com/Detail_...h&lang=eng
(Not sure if linking works here, but if not you'll find them on the Intel site - just search for DN2820FYKH)

Edit: Ah, it seems this might be a larger issue. I do however, seem to have read some people getting HD audio in win8(.1) by making sure UEFI is enabled and Legacy is disabled, in BIOS: And some reported that this had to be set during OS install - changing it afterwards wouldn'y cut it...(?) I might be way off - but I do believe I read this. And likely here in this thread.

Think you maybe getting confused, there was an issue of people not getting any audio over HDMI with Win 8.1 in legacy mode and that is indeed resolved by having UEFI enabled, however there has been no case of either DTS-HD or TrueHD working on Windows due to the missing IME driver which is needed by a Intel & Windows combination for Protected Audio Path (PAP1.5) to work.