I managed to play the Mother's song by ear so I never really understood the point of the song on the wall, other than the fact that the symbols were in the same shape as the glyph that pops up when you play the song. Looking at it now, though, I can see how you can get the song from it.
Hint: Remember that a good portion of the song is repeating the same three notes, so there would be no point in writing them all out repeatedly.
Am I to understand that the ending you get after finding all the memories is basically the same as the one you get without finding them, but with an extra bit on the end? I finished the game with all the memories, should I go back now and finish it again without them to get the other ending?