[pr0xZen]

I have an old yamaha av-receiver that doesn't have hdmi in/out, so i'll connect the NUC to TV with HDMI and from TV to AV-receiver with optical.
In this way i can play all audio format or I could be limited?

Well known situation, myself very recently. As nickr here says, you'll be limited to 2ch (stereo) PCM or compressed formats like DTS and AC3 (As per S/PDIF limitations). Luckily, if your AVR is AC3 (Dolby Digital) capable - an up to date version of XBMC will transcode most incompatible formats for you (AAC multichannel, Dolby TrueHD, Dolby Digital Plus), to AC3.This is not the case with DTS-HD at the moment. But if your AVR is DTS capable, XBMC will pull the "standard" DTS core from DTS-HD, and pass that on to your AVR. If it will transcode that core to AC3, I cannot comment.

Please be noted: The DN2820FYKH (and most other NUCs) does NOT have an S/PDIF connector. You will need a USB soundcard (or DAC) for S/PDIF connectivity. Most of these also have 5.1 analog outputs - and if you use these, I believe you can get TrueHD straight to your receiver (and any other codec - just not DTS-HD atm)... based on the assumption that XBMC can decode these to PCM, seing how it can transcode them to AC3.

Personally I went with a Cmedia CM106 based USB "soundcard" - as these has up to date drivers with good win8.1 support. PM me if you want a link, it's was around $15. I'm not sure the mods would appreciate posting webshop links here... (?)