This old myth of the ducks quack echo being non existent has been investigated by scientist for many reasons. Mainly if the duck has a secret technique to eliminate its own echo then maybe scientists could utilise this in noisy environments where people need to hear clear instructions, such as at a railway station where the announcements of train times are given. At the University of Salford's acoustic research centre the subject Daisy (the duck) was put in an echo chamber and an isolated dead chamber to then have both quacks analysed by a computer program to prove that Daisy's quack did echo.
The scientists did see how the myth might have begun because an aaaccckk sound can mask itself in that the K is stronger than the A and with duck quacks already being quite quiet further the myth.
So there you have it. Another bit of trivia that might win you that pub quiz.