Per "Ask Yahoo!" - We hate to break it to you, but all our sources say a duck's quack echoes just like any other sound. There doesn't seem to be anything special about the noise, except that it has mysteriously produced this persistent bit of folklore.
That intrepid tester of suspicious stuff, Cecil Adams of the Straight Dope, assigned it to one of his researchers. She disproved the myth herself in 1998 with a few friends, a duck, and a large courtyard conducive to echoes. Sure enough, the quack echoed (once they figured out how to make the duck quack, that is). Likewise, the good folks at the premiere urban legend resource, Snopes.com, have personally experienced the quacky echo of talkative ducks.
The MadSci Network suggests an explanation for this common misconception. An echo is caused by sound waves reflecting off a hard surface. Higher frequency sounds reflect better and create stronger echoes. Perhaps some ducks have quacks without a lot of high frequency components, resulting in very faint echoes.
Physlink.com explains how an extremely clever duck might avoid creating an echo at all. If the distance between the duck and a reflective surface falls exactly at one of the nodes of the quack's sound wave, sound will not be reflected back, thus no echo. The duck could also use partially reflective material and some sound wave measurements to prevent a quack from echoing, much in the same way the military hides aircraft from radar. However, we suspect Daffy and his pals aren't quite that sophisticated or conniving.
Only the "induced" quacks of ducks echo... Naturally occuring quacks do not echo, although some ducks do unexpectedly make the sound AFLAC rather than quack, especially is Yogi Berra is around!
Mark
It occurred to me that the duck's quack is reasonably long in duration, if the echo time is short enough it may be hard to distinquish the echo from the source, and the source will "eclipse" part of the echo. I remember sledding with my daughter when she was two at a local country club. The air was still and it was very quiet. If you let out a short yelp (the kind a two-year old generates when she discovers sledding) you could hear a very distinct echo from off the club house about 100 yards away. Since the speed of sound is about 660 ft/sec, the secret was to keep it short.
The most prominent echos I remember are when we watch fireworks from the grandstands at the local high school. The fireworks, high in the air, create a very distinct short report. The school is in the form of an "L" and makes a very nice corner reflector, redirecting the sound towards the stands. The woods behind the school scatter back a fraction of second later so that every report sounds like one loud report, one muffled report a second or so later follow by a low difuse rumble.
Anyway, that's my theory.
Here at the Duck Research Institute (DRI), we're investigating a related question: "If a duck quacks in the woods, but no one's around to hear it, does it make a sound?". We'll keep you informed!
(As an aside, DRI is funded by PETA - "People Eating Tasty Animals". Please support both organizations.)