My cayuga ducks quack all the time, just a constant quiet quack chatter. They're actually quite vocal. They just don't do the loud, "pay attention to me I'm hungry/angry/want something" quack all that often. My drake is constantly chattering with his low, grumbly drake voice. The quiet quack chatter my ducks do does NOT sound like a drake. I've never had a duck sound like a drake, but that doesn't mean it doesn't happen. Just keep listening to his/her voice. It really is the only way to tell. Patience is a poultry keepers best friend.
PS: don't rely on "mounting" behavior! I was so confused the first time I saw all of my ducks mounting each other! I questioned everyones sex/gender when I first witnessed this. They do this as a flock dynamic thing, not just for mating, so females mount females all the time.