Help! Ducks or drakes? Rouen or mallard?.

[Danny_tb]

 

[cleoandtheflock]

Looks like rouens the ones in the back of the picture with the yellowish green bills are drakes

 

[Amer]

Rouens. The two with yellow bills are drakes
The rest you can sex by voice, because I honestly don't trust gender ratios like that. It is much more likely to have two hens, it seems... Isolate a bird. What sound does it make? Is it a clear, loud, quack or a low, hoarse sound?
Hens quack, drakes croak.