My laptop with Windows 10 and Intel HD4600 (also a Nvidea Geforce GTX 850M) stopped playing sound over HDMI at some point. I think it was caused by one of the win updates. I used to be on the Insider program until the fall update. It first happened some months ago, I don't know the exact date (nor update).
At first the sound just did not work at all. I read a lot of posts on different forums and tried different suggested things. However, many of the post were a couple years old, so not addressing the same Windows and driver versions.
Many suggested that the problem is the Intel / Realtek driver in combination with Win10 driver management. I tried different drivers, automatic installation by win10 and manually installing them. I also tried the trouble shooting with win10.
I could get the sound working temporarily with the method of Enghausen in the above mentioned post (in driver settings Advanced tab, change Default Format to "24 bit 48000 Hz (Studie Quality)" ) [with the Intel driver version 184.108.40.20608 installed]. But it only works after rebooting and then stops suddenly, sometimes after multiple hours, sometimes after only minutes.
When I then disabled it in the device properties and re-enable it again, it works only for like 1 second.
PS: I also use Hyper-V (I need to use Docker), since I read about that this was the culprit for some others.