I was having that same problem. I tryed so many drivers version unsuccessfuly. So I decided to install the diver I was using on Windows 7 (15.22.64.2345). And unexpectedly it worked. The sound device just appear when the HDMI is conected. The display driver is:
-Package: 126543
-Graphics: 8.15.10.2345
-Display Audio Driver: 6.14.00.3074
The system will warn you that it's na older version but just install....it really works. And make sure not to install the newer driver through Windows Update. The update will appear and you just need to click with the right button and hide this update.