You’re in the Secret Room (Sector 1 / Main Deck) with the Chozo Statue (bird) and three dormant Metroids (monkeys).
In the Hidden Sanctuary (Sector 5 / ARC), you must play that exact melody on the organ to unlock the path to the Serris Core-X and the Wave Beam.
Write down the melody. The game gives no reminder — it’s a pure memory test rewarding attention.