There seems to be a lot of speculation regarding Momonosuke and Luffy communicating via telepathy and how it’s an application of the Voice Of All Things. Here’s my theory explaining the whole situation.
I’ll break this up to 2 parts, Luffy “talking” and Momonosuke “hearing”.
Firstly, Luffy talking. Luffy is able to send out or “radiate” his willpower to Momonosuke. Based on the sub being able to “feel” it, it seems that this specific instance was a group communication. However, under training, it’s plausible to assume that he can completely single out his communication to Momonosuke, similar to how you can control Conqueror’s Haki. I think it’s simply his insane willpower, possibly amplified by his Conqueror’s Haki, rather than a property of the Voice Of All Things.
Here are the 3 reasons:
1) This is a consistent thing throughout the series. Luffy has unbreakable willpower, and in a desperate situation such as this, he’s able to radiate it, like how Conqueror’s Haki works. Make his presence known, but in the form of a voice.
2) We haven’t seen anyone being able to directly speak through the Voice Of All Things.
What about Shirahoshi
That’s not the Voice Of All Things, it’s her unique power as Poseidon. Also, it’s completely different than what Luffy did. Shirahoshi has imperative command over the Sea Kings, Luffy simply communicates his thoughts.
What about Zunesha
This isn’t a simple Voice Of All Things property, and has to do with the Kozuki clan, and most likely connects back to the Void Century, Minks’ bond with the Kozuki clan, and Zunesha’s punishment. Again, only Momonosuke is able to command it, and it’s also not telepathic. It doesn’t apply to all Voice Of All Things users.
3) The submarine could hear it. This either means a) Technology can decipher “sonar Voice Of All Things” b) Sachi has Voice Of All Things c) They’re humans, they’re close by, and they could feel Luffy’s presence and his extreme desperation. Also, them potentially having Observation Haki could help out.
Option 3 sounds reasonable to me. Another point: They mention Luffy could be heard even though they’re underwater. If being able to pick up sonar radiation is an inherent property of Sub technology, why are they surprised?
This entire line of thinking is further supported by Kaido’s first knockdown of Luffy when he senses Luffy’s intense glare despite him being unconscious. Later on, Luffy unleashes Conqueror’s Haki (still unconscious) and takes out a couple Beasts Pirates. What happened in the water is a similar parallel, except it had a more directed focus of conveying a message and Momonosuke being a Voice Of All Things user was able to fully decipher it.
Now regarding Momonosuke being able to “hear” Luffy’s voice, I believe it to be the Voice Of All Things. None of the other people around him were able to hear it, and the Voice Of All Things gives the power to hear the voice of “all things” aka “everything”, including Luffy’s intense willpower. Otherwise, Luffy would’ve broadcasted it to the entire allied force. Also, his headache reaction was similar to him hearing Zunesha’s voice.
So Luffy speaks through his willpower and Conqueror’s Haki, while Momonosuke deciphers it through the Voice Of All Things. In other words, Luffy speaking has nothing to do with the Voice Of All Things.
*Theory by FirePiyyo