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It's probably not bugged. Something that's potentially not obvious for the upper-left ghost is that the phrase only uses 7 notes, not the full 8. The other thing that might not be obvious is that you can place the pebbles on the bars, not just on top of them (or, in other words, you have access to EGBDF in addition to FACEG). 

Timestamped video solution.

Yeah I was aware of those things; thanks a bunch for the solution confirmation, though. Seems we had the arrangement of all the notes correct, but somehow we never tried to put them at that one specific height, out of all the others we did try. Figures, huh? Gauging the exact pitch of the voice sample vs the sound of the note-rocks was hard for all three of us.

Anyway, great game, loved it otherwise! It's little hidden gems like these that make diggin' around through miles of other indie games of variable quality worth the effort.