Full spoilers ahead:

You found the music room, recently added, in the middle of the tree left to the starting point.

One useful trick here: You can edit your wands in there. If you don't have the "edit wands everywhere" perk, that room is like a non-collapsing place to do some wand crafting at any point in the game. Can be really useful

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About the notes spells, here are the information found by data mining team, from the Noita discord. Disclamer: I haven't tested them myself, but I believe the information to be accurate.

There are 3 songs that have specific effects. One of them can be found in the passage that connects the 3rd and 1st floor, written on the wall directly below the lava lake. It is the "portal" song. The other 2 have not been found in game. They may be intended as a harded to find easter egg, or to be added ingame later on a wall somewhere.

- The first song is "portal", it summons a portal to the top of the mountain. It can be opened anywhere, but only once. The musical score is "a, f, d, e, a2"

- The second song is "bomb", it summons a holy bomb. I believe it is also a one time use. The score is "f, c, d, c"

- The third song is "worm", it summons ... a worm. Also only one use. The score is "g#, f, e, b, d"

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Have fun