If you have not formally selected a sound device as the default path for output, the DisplayPort/HDMI digital audio path will be made active when the monitor and capability is detected. If you go into the Sound Settings and select the RealTek HD Audio device as your default, it should continue to output there.
Hope this helps,
Thanks Scott. I tested the NUC today, and it works OK. It seems the problem I have is with the interface to my smart TV - when the HDMI port connected is active. it switches off the PC audio port. Thus no sound! I'll live with this at the moment.