Luffy drowned at start of Chapter & few minutes passed until Momo started having a Headache so i doubt it's that.
I think it was because they reached Wano.
Someone might say that Momo has always been in Wano but never heard that Voice so why now?
I believe the difference is that they are high in the Sky now
I think the Source of what Momo Heard is either in the Sky on top of Wano or it's on top of Mount Fuji
It is linked to what Sengoku said to Akainu when he told him that Wano attracts Big Shots, as if something special is hidden there
If you check the Map of Wano, you see that most of it is Submerged in Water, so imagine when it wasn't, Mount Fuji would have been even higher & higher & could have reached the Sky once. So imo there is definitely something hidden up there
It will also be an opportunity for Momo to carry Luffy and/or Zoro and/or Yamato to location of Third Round which is in the Sky.
Kaido's Introduction is another Major Hint showing that Kaido is aware of existence of Sky Islands & it makes sense that he hid the Poneglyph up there since he is a Dragon & found that place
If you think about it, Kaido doesn't care about crushing Capital with Onigashima & apparently Law couldn't find Poneglyph there & also in all Wano other Reigions, there isn't enough Guards to suggest there is a Poneglyph hidden in one of those Regions (Actually outside Capital, all of them are near abandoned)
So i'm 99% Confident, Poneglyph is in Sky Location on top of Wano & Final Round will be there & that is the Source of Voice Momo is hearing right now