Julie, til they are really starting to feather out, unless you are a seasoned pro you will not be able to tell the difference by voice, they make virtually the same sounds
girls will be more vocal than males as always, but the voice is just a baby duck voice, very hard to tell 100% by it til 6-8 weeks.
I was getting some calls a few weeks ago, and the breeder swore they had them figured out by voice...LOL we spent about 2 hours changing all her bands for sex ID after I started vent sexing them for her, just goes to show, it doesnt work til they are near adult hood and making 100% possitve Quacks.
Some may have little tricks, that breeder did too, they werent accurate.
If it's a big deal to know now, the easiest way to know and know without a doubt is to vent sex them. It's very easy to do. I will be happy to walk you threw how to do it if you'd like, or I think there is a sticky topic at the top of this section about it too.
Once you do it one time, you see just how easy it can be. May take a few swings at it to get it, but once you see what you are looking for on a boy one time, you'll have it down.
No on ducks and chicks, there is now real one sex more dominate in the hatch than another. Some say temp can play a role in it, I feel it's just luck of the draw, the DNA codes are already in there upon fertilization.