The aforementioned bonus room has its own song, a lo-fi track accompanied by guitar with the lyrics written specifically for the level:
Here's a little bonus room, because I know you've had it tough.
Here's a little bonus tune, about collecting real cool stuff.
Yes, here's a little bonus room, where you can play.
Don't be frightened, don't run away.
The complete lyrics can be found in the related links section. Add