This message was posted on behalf of Intel Corporation
Thank you for contacting Intel Communities.
Seems to me that this is a compatibility issue with the OS you installed.
Since this is a relatively new hardware, which supports Windows® 10 only, it may take a while to get a 100% functional Linux* based graphics driver. (audio over HDMI is part of the graphics driver)
At this point, my recommendation is to run the system with a fully supported OS.
You can also go to 01.org to get more information about Linux* driver support
I hope this information helps.
since I'm experiencing the same issue (under Arch Linux) I've filed a bug over at freedesktop Bugzilla:
I hope you don't mind I've used your report text from: ALSA User — Intel NUC7i5 only getting stereo on HDMI