Mermaids have human-like breasts, but those should never have evolved since it makes them less streamlined and hydrodynamic. Ever wonder about the function of those?
Turns out that just like a female human's breast, a mermaid's breasts contain a high amount of fat - which is not a bad thing in itself. The "melons" of a mermaid work just like the melon of a cetacean!
In another question I described their respiratory system as having something I called gillungs... Those are kinda like branchial chambers with internal gills, and those gills could vibrate just like vocal cords. The breasts of the mermaid can then function as resonating chambers, so that she can sing like a dolphin. Yes, that luring song they sing comes not from their throats, but from their chests (in fact, that's very close to how parrots speak - they don't have vocal cords either, their vibrating organs are located quite closer to their lungs).
And that's how mermaids communicate among themselves - just like whales and dolphins do. Your mermaids may learn how substitute their gills for vocal cords, and how to make strange sounds with their tongues and lips in order to communicate with humans, but that may be unnatural to them.