I would recommend loading the HDMI 2.0 firmware update. Please give a try. See this link for downloads and instructions: https://downloadcenter.intel.com/download/26061/HDMI-2-0-Firmware-Update-Tool
I have done that but still I don't have any option in win10 for 5.1 or any kind of surround. The only option I have is stereo.
Thanks for the update.
Please make sure you have loaded the following drivers:
Latest Intel® NUC BIOS version 0042
Realtek ALC* audio driver version 126.96.36.19982
Graphics drivers version 4552
Hope this information is helpful.
Please do let me know if you need further assistance with your Intel product, I will be glad to assist you.