P.S - I am on latest bios and latest drivers, and running Windows 8.1, Ethernet (So no wireless) if that makes any difference.
Here is also the link to the sound that my speakers make with Dolby passthrough on. This only happens with NUC. With my old HTPC that hooked to the same setup over s/pdif, I never had this issue.
Daneel3001, we always recommend customers to use straight connection. In your case, the Onkyo TX-NR609 AV receiver is working as an adapter. That could be the reason of the behavior you are getting when it is between the NUC and the TV.
We tried to make as many third party components compatible as possible, but do to the sheer quantity of these third party components we cannot always guarantee compatibility.
Are you getting this sound when using straight connection?
Please try using another HDMI cable 1.4a in order to test if it could be defective.