This is probably because I don't have oss emulation enabled in the kernel. Anyway, it was solved just by deleting the /opt/nwn/lib directory.
Is it possible to add a new use flag that makes nwn use the emul-linux-x86-sdl package instead of it's own version?
deleting /opt/nwn/lib also helps if your sound is distorted!
Personally, I think it is a bad idea to not use the bundled libraries. They will help upstream if someone ever decides to post to their forums for help. This is the main reason why we always use the bundled-libraries, where possible.
Closed. I also reported it upstream.