Curate, connect, and discover
Imagine if Whole Cake Island was a full on musical except Sanji is the only one unaffected by whatever is in the air so he goes around sulking & non harmonizing & being annoyed with everyone because he's the only one aware of it. Not only would it be hilarious, but it would further emphasize how much he isn't supposed to be there. How out of place he is.
Even the strawhats start singing when they hit Totto Land waters. Luffy has a whole song about how hungry he is after he decides to starve himself in protest called "meat" but it's actually a metaphor about yearning for the return of his Sanji. Nami has a whole villain esque song about taking advantage of the Homies & Zeus. Chopper & Carrot have a cute little tune about being in dessert heaven.
There's even a big multi POV song about everyone getting ready for the wedding & how they're actually getting ready for their respective ploys—murdering the vinsmokes & attempting to stop said murder. Sanji is flabbergasted because how is no one else hearing everyone's true intentions? You're literally belting your scheme step by step next to your enemy! Whatever. Guess this is normal here. Anyone who tries to pull him into a song gets a kick to the face.
Brook actually also knows what's going on, but he's in his element and having too much fun so he doesn't say anything in. He has the best numbers in the entire arc & the musical quality of the islands actually give him a powerup during fights. The better the performance the more power radiates off his body.
His power ballad about wanting to see Big Mom's panties is so powerful it's actually able to be heard by everyone in Totto Land. They all stop what they're doing to vibe & enjoy the killer guitar solo. Even the Vinsmokes & Big Mom Pirates pause being assholes to enjoy the song.
When the crew is leaving Big Mom's territory they suddenly become aware of all the singing they did & Sanji tells them how weird it was. Brook accidentally lets it slip he too was completely aware & so Nami beats him unconscious for not telling her. They agree not to tell the others in Wano & make a pact to take it to the grave. What happens in Totto Land stays in Totto Land.