Okay then - I have to tag along on this thread and ask a question of my own.
I got two Mallards this year too, Rainstorm and Lullaby. Rainstorm is definately a drake - the bill color is right, the beginnings of a drake curl have been evident since he was
very young, and he's got his hoarse, whispery boy voice. Lullaby's bill color is muddy - it might turn yellow/green, but it's got a orange/brown tinge to it too, and she sounds like a drake
most of the time - hoarse and quiet just like Rainstorm. However, if we take Rainstorm away, out of Lullaby's sight, Lullaby "QUACKS!" good and proper like a girl duck.
They're only two months old, so I'll have to wait awhile yet to see who develops male plumage, but, according to what I've read, drake's vocal cords are strung differently and they can't make a loud quack like a girl - so Lullaby has to be a girl right? Maybe she's just mimicking the sounds Rainstorm makes most of the time and that's why she sounds like him. Maybe I have a hermaphrodite on my hands!