I had a similar problem with a nuc5cpyh. Although Volumio had spdif as a possible output there was no sound.
The fix for me was very easy. Get to a terminal on your NUC, type alsamixer and then unmute spdif. Leave alsamixer with esc.
In my NUC this is the output of aplay -l
volumio@********:/tmp$ aplay -l
**** List of PLAYBACK Hardware Devices ****
card 0: PCH [HDA Intel PCH], device 0: ALC283 Analog [ALC283 Analog]
Subdevices: 1/1
Subdevice #0: subdevice #0
card 0: PCH [HDA Intel PCH], device 1: ALC283 Digital [ALC283 Digital]
Subdevices: 1/1
Subdevice #0: subdevice #0
card 0: PCH [HDA Intel PCH], device 3: HDMI 0 [HDMI 0]
Subdevices: 1/1
Subdevice #0: subdevice #0