Well, I got the game running in compatibility mode, but the sound in the starting movies stops every 5 to 10 seconds for a moment. Then goes on. As far as I can tell so far this is not happening in the game - so it's not a really severe problem, but still. I've tried everything from nwnconfig.exe to the ingame sound options and it got a bit better but it's not gone. Anyone another idea?
P.S.: Talking about NWN 1, by the way.

I had the same problem. It hasn't effected gameplay though, so I never bothered to try and fix it.

Yes, it's not a big thing, of course. If nobody has any new idea I guess I will simply live with it - like you. But it's sort of nice to hear that someone got the same problem. ;-)

Try lowering your sound sample rate in control panel:

Control Panel >> Sound >> Playback >> Speakers >> Properties >> Advanced...
In the drop-down menu, if you have 24 bit sound, reduce it to 16 bit 48000. Also make sure both 'Exclusive Mode' buttons are checked.

Someone just, within the last few days, posted that DX11 has sound issues. If you're using it try installing DX9 over it. It won't over-write your DX11 but it may add some missing files that will fix your issue.

It was already at 16bit 44100. Both buttons are checked, so that's not the problem. Also it is the only game so far which shows this behaviour, and I have meanwhile quite a few games both from Steam and GOG. Hm, I'm not exactly sure if I would want to try this one. Where would I get a DX9 nowadays? And wouldn't that problem show up with other games, too? Or even with the ingame-sound of NWN1?

All it does is install files that are missing from the newer version. It has no impact on your DX10-11 install.

Straight from Microsoft. To be clear DX9 is a parallel install to DX10+. Which is why nothing is overwritten.