Looking for a big clue? Go to the last paragraph.
I got horribly stuck for a few minutes. I didn't read the game description or notice the hints changing, so I got to level 5 without realising I had any new abilities beyond starting/stopping the music and being able to use my lives on the monsters. (Level 4 involved some jumping while .)
Finally, I noticed that the text at the top had changed, then read the game description to realise what was up.
Nice little puzzle platformer, very meta!
THE CLUE (SPOILER): every level becomes incredibly easy once you realise that *every single level* you get a new "power". The powers are used by clicking on the right things, and the text at the top of each level is a hint about what to click on.