It's kinds of a loud raspy quack. Mine is only like 5 weeks. I posted a video to a duck FB page I had one say boy and 2 say girl. I my Pekin quacked early and my khaki Campbells are girls. I can't remember when they quacked though. I'll see if I can upload my video.
I know this is an old thread but I just saw this and joined. Did these turn out to be boys or girls? I have one that has a greenish tint on its head like your pictures and I'm hoping it can still be a girl. It's around 5 weeks old. Thanks!