The piano keys real order is:
C D E F G A B
You can find it in one of the previous rooms, where you meet the "bugged" girl, it's on the south wall on a blurry picture.

In the room with the piano, you'll find a note containing the number codes for each key and the fake key order.

You have to press them in this sequence:
B D D F E C
(C makes no sound, serves as an "enter" key).

You'll immediately be attacked by a crawling monster, so be prepared to run or shoot. :)