You may want to consider updating your BIOS and see if the audio works when connecting to a different device; for instance, connecting directly to the TV instead of the AVR. You may also get support from the Linux* Community.
I hope this helps.
O.K.! BIOS is the latest, of course. NUC is directly connected to Panasonic Plasma TV. Nothing changed. With the upcoming Ubuntu 14.04 it should work everything fine. If not, i will be really curios about Intels further statements...
Dont forget: The NUC are officially "Linux-Compatible". So i really would appreciate, if the Linux Support outside of Kernel-Developers is getting more than a fashionable phenomenon.. Thank you anyway!