Thankfully I have some babies now. I have three ducklings that are six weeks old. The female has been quacking for a couple weeks now. The other two, I assume are males, still don't make much noise. In case you didn't know, the females are the ones who make the quack quack noise you always think of as duck sounds. The males have a quiet, raspy voice.
I've also noticed that with ducks like mallards, every mallard I've had, by about a week, sometimes even at hatch, the males are bigger than the females, I'm not sure if thats always set in stone, but with all the mallards I've had, thats how it's been.
I have mallards when they were only days old they were the same size but when weeks started coming the boys got bigger and I started to hear the little ones quacking So with me also the bigger ones have always turned out male...