Females Quack......and the male well.....the male duck he sounds like a dying cat or somthing......extremly different, you will be able to tell really easy + the drake feathers.
Edit: my drake sounds like a dying cat......thats why i said that lol
I think what your saying is the male sounds like he is quacking through his nasal cavity, and it sounds like when a person speaks with a nasal tone....?
As has been said you'll be able to tell about 5-6 weeks old when the ducks start quacking (especially when you pick them up). They will quack just like you'd expect a duck to quack - you won't be able to miss it. The drakes will keep peeping but will eventually start to make a husky, rasping deeper sound. The males will eventually develop a slight greenish tinge to their bills (even the black ones) while the ducks will not.
Your blue one with the yellow round the bill will end up with a bit of white round the bill. With Swedish you can tell what their markings will be like from a very early age.
The black one we are pretty sure is a female. She is already trying to quack and she has a loud clear voice. The small blue is up in the air and the big blue iwe think is a male. He is larger, raspy sounding and slower developing.