You will begin to recognize females voices, quacks are females. But the silent ones are hard to sex by voice. My son raised three pekins from "day old" ducklings last year. One soon declared herself to be a very noisy little girl. She is crested and smaller than the other two, but makes up for it with very loud quacking. The other two were very similar sized as juveniles. One got a drake feather; one didn't. Both were virtually silent. Who needs to make a noise when you have a very noisy crested duck in the family? This April when the drake was 13 months old, he was rehomed with my drakes. The previously virtually silent duck has taken over as guardian of the flock and she now makes very lound quacking noises. It took her 13 months to quack! Of course we knew she was female as she started laying at 19 weeks and has laid daily since then.