As I mentioned before, the girls will hiss, when they are upset, feeling threatened, or protecting their ducklings, nest etc. The boys, once mature, vocalize by Hissing and they ... hmmm... how to put it... take a deep breath and blow out their cheeks....
I don't know how to describe it but you will understand when you see it.
The boys will never coo or chirrup once they lose their baby voices. The girls/hens will coo, chirrup, hiss when angry or upset. Don't worry, it will become very obvious to you soon and it gets easier to tell them apart earlier w/practice, observation & experience.

