The chicks we have had started sounding chickenish around 14 weeks, but I bet that is veries by breed and personality. I think that I read a post from Speckledhen saying she had a chick try to crow crazy early, I want to say under a week old?
I had a banty rooster start crowing a 6 weeks last year. In my experiance the banty's mature faster than standard breeds. I would say my standard breeds started to make "real" chicken sounds at about 13-14 weeks. It's been years since I had ducks so I don't remember a whole lot about them. Good luck with your babies! I'm jealous over your ducks