I have 2 little drakes that we swore for the longest time were a pair because they were the same age but 1 was so much bigger. They are now about 2 1/2 months old & are just coming into their adult feathering. They are starting to get VERY green around their faces starting at the beak & working slowly out. They have always had very soft voices. The first feathers I noticed a difference in were the tail feathers tho. Their tails were rimmed in white & steadily got lighter tan. Now they are getting the greyish feathers at the edges of their tails on their bodies & their bellies. The underside of their tails are also turning grey.
The 2 rouen/call mixes that I just got yesterday are definitely male & female. They are about 2 months old & the female definitely has a much louder, harsher quack. The male now has a hoarse peep when he quacks. He still sounds much like the ducklings only with a bit of a rasp to his peeping. They have the rouen coloring, which is pretty much the same as the Mallards, but neither are showing any feather changes yet. There is just a hint of white around the drake's neck, but that may be from the call parentage.
None of the 3 drakes have their drake feathers yet, but the 2 Mallards look like they each have a feather or 2 that is thinking about curling. They are still pretty early into the molt tho.
I will try to post pix later.