Hi again!
I was almost frightened for a few minutes ago when opening the editor - and suddenly "Positive force" popped up out of nowhere (It's great music, just totally unexpected). Especially as I don't even use it in my test level.
After some investigation, I came to the conclusion:
Somehow, VVVVVV keeps track of your last coordinate, and starts some music based on that IF, and only IF, that coordinate triggers a music file in the original game.
For example, I played around in 10,1 in my own level, saving, quitting and went into the editor - and Positive Force started to play. I realised 10,X is where The Tower is.
Tried to play with this, and placed myself just right where The Laboratory begins (that labyrinth in the start), and then fired up the editor again. The Laboratory music didn't start to play as expected - but Passion For Exploring did. So clearly there's something related to this.
I can be wrong though - but this is completly reproducible for me.