BillyBob is giving you the correct method of repeating what the organ plays, but I found it more helpful to look at the blocks of light, rather than trying to play from the keys. Clicking anywhere within a block of light will play the chord. So your pattern will be: SPOILER
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O=organ; Y=you
O 1, 4, 3
Y 1, 4, 3
O 1, 4, 3, 7
Y 1, 4, 3, 7
O 1, 4, 3, 7, 6
Y 1, 4, 3, 7, 6
O 1, 4, 3, 7, 6, 5
Y 1, 4, 3, 7, 6, 5
O 1, 4, 3, 7, 6, 5, 2
Y 1, 4, 3, 7, 6, 5, 2
Puzzle solved!!!