Varroa, the resolution needed in order to access the Intel® Visual BIOS Setup when pressing the F2 key is 1024x768.
Could you try another display in order to find out if the issue persisted?
Please check the HDMI cable you are using is 1.4a, since this is the only one that support audio and video.
Just in case, this is the latest Intel® Iris™ and HD graphics Driver for Windows 7/8/8.1* 64
I presume this has to do with power saving features of the NUC...
For me sometimes the sound does not come up either(while the video playback is around 3 fps), but it is solved by pausing again, waiting a sec while it connects and resuming the video...
Thanks Sylvia, but as I said the resolution issues seem to be fixed since the last bios update. The sound cutting out probably has something to do with the power settings as Dr.Romca has said, I will investigate further.