bjork said:It changes later on, after several playthroughs. Pretty cool stuff.
The music changes depending on the mood of your Nightopians in the levels. If you pamper them and they like you, the music gets more cheerful. If you kill a lot of them, and they fear you the music gets darker.
On the Christmas Nights CD you can check how the Nightopians in each level feel, and also change the settings for the music playback (only in that analyzer, not for the actual game).