Can't get surround sound on new Intel NUC

[Chris Adams]

I have a new 7th-gen Intel NUC (NUC7i5BNB), hooked up to an audio
receiver via HDMI.  I have tried a bunch of different things, but I am
only able to get stereo audio out of it.

I can connect my notebook to the same HDMI and get surround just fine;
when I select that output, the receiver sees a PCM bitstream, but it
only ever sees stereo from the NUC.

Both systems are up-to-date Fedora 25.
-- 
Chris Adams <linux(a)cmadams.net&gt;