My older ducklings are 5/6 weeks now and they have a huge array of sounds they make. The runners like to sit at the waterer and squeak*mumble*chirp*cluck to themselves. The whole group makes sounds like quacks, shrieks, sqwaks, whistles, trills, clucks, mumbles, something that sounds like buhbah. They are supposed to be all girls. They are all incredibly loud when they think its treat time.
What have you noticed about your own birds?
Incidentally, when are they no longer considered ducklings?