This is nothing to LOL about!
Well if this "Azure Flute" is a legend, or at the very least a rumor, then I think that it would be best for it to be an angel or some (demi-)celestial or (demi-)divine being; someone who, for some reason, has been shown to come in contact with Arceus in the past or has shown some unusual potential or behavior that was unorthodox for humans.
Their voice could still be the way they gain access to the Hall of Origins. The Azure Flute, as the game puts it, "[echoes] sounds that do not seem to be of [the pokemon] world. Someone with an exceptional and surreal voice would truly be worthy of the title "Azure Flute".