I've got the same issue with my NUC. Oddly enough, movies and TV from my NAS work fine, but music, PVR TV, and system sounds do not work. I thought it was related to stereo audio, but 5.1 AC3 from my HDHomeRun isn't working either.
On another note, internet streaming, ie YouTube, NBC Sports etc. works intermittently... probably only 20% of the time.
Edit: Problem solved. My wife inadvertently turned the volume all the way down using the Yatse remote and so, obviously, passthrough audio was the only thing producing sound. That's embarrassing.