I know nothing about duck calling, but let me tell you what I know about turkey calling that I bet is the same:
-you are more likely to call too much than too little
- ducks (turkeys) are some of the worst callers out there, the noises you make need to have the right "tone", rhythm, and be right for the time and place - the pitch and prettiness of it are the least important because the real birds can sound horrible to the point where you will swear there is a tone deaf hare lip the next ridge (pond) over practicing their calling.

As far as practice/ examples, and again, I know nothing about duck calling, but I bet there are some good samples for free on you tube to check out. (But cd's from manufacturers are out there like the old ML Lynch turkey call practice record).

However, if anyone who knows more than me (which means anyone) wants to say I am wrong, do not doubt them.