You should raise an Exception instead of returning false for invalid inputs.

Fun kata (coming from a fellow guitarist). There's really no practical distinction between labelling strings 'e' or 'E' for the purpose of decoding notes vis-a-vis frets, though it adds another elements to the task at hand.

Please, try to properly format code when posting, with a triple backtick before and after it.

That said, your code does not pass because it is extremely inefficient for this task and there is at least a better way to get the result :)

Expected: [], instead got: ["wha"]
I'm at a loss as to how to pass this test without knowing the input...