There always is a root, some OS protect it so that you can sudo only though 😉
Could you type
ls /opt/etc/mt-daapd/mt-daapd.conf -al
And post the output? It would show who has which access rights for that file…
Or you could immediately change them, I guess since there are no other users on that NAS, it isn’t that bad to make it available to every user on the machine:
sudo chmod 777 /opt/etc/mt-daapd/mt-daapd.conf
sudo tells the machine it should do the following command with superuser rights (aka root rights).