I've now played the Aquaria demo for one week.
All I got is one song (shield) and 0ne plant (green to atrackt yellies).
I found the song cave and the four stones which you can tell you the song (hope in correct sequence). I've sung this song I think 100 times before the grey door. There is not tutorial where to click or what button to use, that it will open.
Also your short beginners walkthrough gives no solution.
Son any hint, what I have to do?