Just wanted to update this thread in case others down the road are referencing the same question. At 9 weeks mallard developed her quack. Late bloomer. Also heard in video is 4 wk old Pekin who found her quack at 2 weeks. Mallard is definitely a girl.
Here is video of sound only of my 7 week old duckling. I’ve heard only handful of questionable loud quack sounds in past few weeks. This duckling very quiet. I had to separate it from it’s friends to get this sound from it. Beginning of video you’ll hear a definitive quack in background but...