what do i have to type into the console to make it play cd-music?

Player Name in Nexuiz: Svck my Bolls

you mean from the fake CD or like replacing the music with CD music? for the fake CD youd type 'cd play #'

As i understand it Nexuiz WOULD play your CD if you pop it in the drive IF if would not have the 'fake' tracks..
So i think you would have to remove them (sound/cdtracks) from Nexuiz/data/data20070531.pk3 then it would work. Or maybe it would be worth to pack them into a single pk3 in the future like some linux distributions do..