Voice sexing 9 week old ducklings?

[KikiDeAnime]

I was wrong! The person selling them said that one is now quacking so it's definitely female. I'm going to buy them despite the other being male.
Both drakes will be kept separated unless I can somehow get them to like each other and not kill each other. If I get both drakes to be okay with each other, the females will be kept with the chickens until I breed them.
Plus I can breed the Runner drake to Duchess for little Runner ducklings.

 

[Magnolia Ducks]

Runner ducklings are perfect.