Male ducks or drakes sound like bull frogs lolI had ducks that I didn't know if they were male or female and it wasn't until they were over 3 months that the drake feather grew in. Here is a link to examples of male and female quacks:
http://aducksaysquack.wordpress.com/2009/08/04/sexing-ducks/
It's fun just to hear them!
Jen