I've played with a lot of the solutions. My installer originally sold me on Sonos for distributing through my house. I've tried other solutions since then, and Sonos is actually pretty clunky in comparison to their alternatives (IMHO). I think it just got there first and establishd a name...
It's a USB to Toslink adapter. You'll plug it into a USB port on your NUC (more likely a mini-USB hub) it will enable digital audio out. Connect from that to your sonos. Video can stay going to your AVR, toggle the Toslink adapter as your default audio device, and your audio will come...
There are quite a few receivers that will output, or do passthrough digital. I'm not sure if that one will. You will need a USB to Digital (Toslink) adapter like this one:http://www.turtlebeach.com/product-detail/sound-cards-accessories/micro-ii/31