How Joyboy Gained The Voice of All Things!!
It's actually simpler than you think, but it takes a big leap of logic.
Joyboy met Poseidon, Pluton, and Uranus, and they taught him how to use the Voice of the Sea, Voice of the Land, and Voice of the Sky respectively. After gaining all three, he gained The Voice of All Things because everything in the Land, Sea, and Sky is literally All Things in the world.
The Leap of Logic
Poseidon, Pluton, and Uranus were real Gods at one point, and they were the Ancient Kings of the Fishmen, the Minks, and the Bird guys.
Evidence (?)
Luffy inherited The Voice of All Things from Roger who inherited it from Joyboy. Luffy has previously demonstrated that he can hear the Voice of Sea Kings and Zunisha. The Sea Kings and Zunisha are Aquatic Animals and Terrestrial Animals that only listen to the commands of the Kings of their territories. They only listen to the person who has the Voice of the Sea, the Voice of the Land, and the birds only listen to the person who has the Voice of the Sky (but we haven't seen much of that yet). Luffy can HEAR them all, but he can't give them all orders because they don't belong under his command. Nika is the Sun God. Poseidon, Pluton, and Uranus are Gods of the Sea, Land, and Sky. The animals in their territories only listen to them, but they're all aware of who Luffy/Nika is because they met him before when he met their King
As a final note, I've been explaining this to people for years, but they all told me to wait for Oda to explain it. I'm tired of waiting for Oda to explain things and he usually does a terrible job of it, so I'm taking it into my own hands lol
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