You looked that up in a guide! achievement in Call of the Sea

Play these 5 notes on the keyboard in chapter 3 in this order (after turning on the power):

392 (5th white key)
294 (2nd white key)
466 (Last black key)
523 (Last white key)
440 (6th white key)

You will know you did this right if Norah hums the tune after you played it.