The following information comes straight from a Nintendo customer service rep. There are some spoilers involved, so I've split the details into two sections. The main spoilers come after the jump.

We have been made aware of the issue that results in being unable to progress in the Song of the Hero quest. If the game has been saved after completing the sequence of events, it will no longer be possible to proceed through the game on that save file. The only options at that point are to either start the game over or to use an earlier save file to continue.

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