I have 3 right now that are 10 weeks. Honestly, I can't tell the difference. I looked at your pictures, and I "think" that I could see the green sheen on your duckling's head... I am not sure though... they all look the same to me. I stink at this!
2 of mine say "WHACK, WHACK, WHACK, WHACK, WHACK!" Or, another way to describe the sound is like a "Ah! Ah! Ah!," like a very loud laughing sound. One duck says whack/ah more than the others, but two of the ducks say whack/ah an awful lot and terribly loud!
The other duck doesn't really say much. When it walks he sort of just says "heeee, heeee, heeee." Unless it is separated from the other 2 ducks. At that time it will make a loud sound... similar to the other two ducks. It could just be a very quiet hen....
So, do you think that I will be able to tell - Do you expect them to begin molting and growing in their adult feathers in roughly 2 weeks?