Hello Sminten, thanks for joining the Intel Community.
There are some audio settings on the NUC that you can try. Please check the following:
Go to Start > Control Panel > Sounds and Audio Devices, click Audio tab. Check the default device for sound playback; there can be multiple selections. Select the desired device, either rear panel or front panel.
Besides this, there are no settings on BIOS that need to be change or any additional driver that needs to be installed.
I have the same problem with my Vizio E700i-B3 TV connected via HDMI ARC to my Vizio 5.1 sound bar. I think the issue is that the TV reports its default configuration (2 speakers) rather than the configuration of what it is connected to via ARC. It would be great if the Intel driver allowed you to override the default reported by the connected TV.
Only other option appears to be getting a USB sound card that support optical out and connecting that to the sound bar. One technology step forward, 2 steps back