This is awesome! Been waiting for a decent musical easter egg like this for a while. The piano in the saloon in Buried could be played if you shot the keys, but was very limited with what you could play. Last decent musical easter egg I remember is Tifa's piano from Final Fantasy 7 where the piano had a great range and you could jam portions of many songs.
For anybody interested, the notes you are playing from left to right are D, A, D, F, G, D. I don't know how much the pitch changes between each bulb when you add/remove water, but if anyone had footage of each bulb with notes being played at each of the water levels, it would be easy to ascertain all the notes that can be played with each bulb, and which other zombie songs, if any, could be played.
Edit: First bulb/note is actually C. I was going off of 'Samantha's Lullaby' from the Moon loading screen, and I am still convinced that the note in that piece of music is a D. Strange.