[noggin]

Also I have another question about audio playback.
I have my NUC plugged onto an AVR receiver, which decodes all HD audio codecs for videos.

It won't decode "all HD audio codecs" - it will just decode the mainstream consumer HD audio codecs (DTS HD Master Audio and High Resolution and Dolby True HD) along with the legacy DVD-quality audio codecs (Dolby Digital and DTS) (There are also a few other variants that are less widespread like Dolby Digital Plus and some of the Dolby/DTS enhancements like DTS ES which are also likely to be decoded) (A few amps may also support some flavours of Windows Media Audio I believe?)

However there are other codecs using the computer (and broadcasting) world that are not usually (if at all) decoded by consumer amps - such as AAC, FLAC, OggVorbis etc.

When it does this, I can see that it decodes an HD audio stream by a little logo appearing.

Yes - because your amp can decode bitstreamed Dolby True HD an DTS HD MA/HR it signals that fact.

When I play FLAC audio files, the AVR does not seem to change anything, and thus I'm wondering how the NUC is sending the audio stream to the receiver (which seems not to see any HD audio stream but a "normal" one).

FLAC (and AAC) will be decoded to PCM (in some cases PCM multichannel - which can be carried via HDMI and is supported by the Intel GPU audio device) and fed over HDMI in PCM. As XBMC will default to PCM output when not playing a bitstream track, then your Amp won't know that it is being fed anything different unless the FLAC is multichannel and/or a different bit depth or sampling rate and so won't display any changes.

However - your amp itself is doing the same process when fed DTS HD or Dolby True HD - it will be decoding to PCM (multichannel in many cases) within the amp - all that XBMC is doing with FLAC is doing the PCM decode itself as the amp can't.

I chose the i3 instead of the Bay Trail in part because of the HD audio chip, but now I'm wondering if it is usefull only for video playback...

AIUI the Haswell and Bay Trail NUCs have the same functionality in HDMI audio terms - but there are Windows driver issues at the moment with the Bay Trail and HDMI bit streaming of HD audio (though OpenElec and Linux distros are fine)

Not sure what the HD audio chip you refer to is - could it be the analogue audio functionality (or SPDIF coax/Toslink optical output)? The HDMI audio is handled by the CPU+GPU integrated chip AIUI.

(Most modern PCs have two audio devices. The HDMI Audio for bit streaming "HD Audio" and the "HD Audio" chipset for handling analogue and optical/coax SPDIF audio output - and they are two unrelated and independent sections of the PC.)

Is it normal or am I missing something?

It's normal. Your Amp can't play FLAC natively, so XBMC will losslessly decode (no quality loss) to PCM rather than your amp doing it.

(** PCM is the standard, uncompressed audio format carried over HDMI. You can think of it a bit like WAV. There is no compression at all. FLAC, DTS HD MA and Dolby True HD are all losslessly compressed formats, where the original PCM audio is carried with lossless compression. Think of it like a WAV file that has been put into a ZIP archive. Consumer AVR Amps can decode DTS HD MA and Dolby True HD to PCM internally - so you can send these signals bitstreamed over HDMI. Consumer AVR amps can't decode FLAC so XBMC has to do the PCM decode. All that is happening is that the decoding is taking place in a different point in the chain)