I can see where this urban legend could
When something makes a noise, say a gunshot in a canyon, the observer hears the noise echo around. This is because the sound waves take different length paths on their way to the observer. So you hear the bang and then you hear it again when the sound has reflected off a big rock wall and come back at you.
Ducks often live in marsh land or on lakes and I'll stick my neck out to suggest this is more common than finding them in mountains or canyons. So if the duck is in a large open space and it quacks you don't hear an echo because the sound that's travelling away from you has nothing to bounce it back in your direction.
I'm suggesting that people hear the echo of a duck's quack less often than they might hear echoes of other noises.
On the other hand, maybe ducks don't emit sound waves at all. Maybe human's have evolved a psychosomatic reaction to ducks that actually manifests itself in the form of vibration in the ear drum, so the sound we hear is generated within us, avoiding the need for it to travel from the duck at all. That would remove the potential for it to bounce off other surfaces in the environment on the way to our sound receptors.