Can anyone say, for certain, that a duck's quack does not echo... and
no-one knows why?

Thanks for asking!

No, no one can say for certain, because a duck's quack apparently DOES
echo. Some evidence? You bet!

Barbara Mikkelson of Urban Legends at Snopes.com (quoted by the likes
of ABC News, and CNN) debates this claim based upon personal
experience.

"I'm simply going to dismiss this one from personal experience.
Although I grew up in suburbia, much of my youth was spent raising
various kinds of domesticated animals, particularly ducks and geese.
When those ducks got to quacking, I could most assuredly hear the
cacophony of sound as it echoed off the stone walls that surrounded
our yard and entered my bedroom window. So could the neighbors a few
hundred feet down the street, who frequently called us to complain
about the noise. The surprise was not that our ducks' quacks didn't
echo, but that they echoed so remarkably well."

Urban Legends - Snopes.com
Ducking the Question
http://www.snopes.com/critters/wild/duckecho.htm 

Cecil Adams of The Straight Dope website debates this claim based upon
empirical tests.

"Finally Shareen had an inspiration. She held the duck by his body so
that he could flap his wings, and ran up and down the length of the
courtyard hoping to replicate the experience of flying. So much for
being discreet. Incredibly enough, this wacky stratagem worked. The
duck loved it and quacked like crazy for a minute. Yes, the quacks
echoed. This was heard by the three of us and by an unidentified East
Lansing High School teacher who came out to make sure we weren't
engaging in duck torture. I was able to record the event but didn't
get a good sound recording of the echo itself. But I do have a dandy
clip of Shareen running up and down with the duck. I call it my 'duck
tape.'"

The Straight Dope
Is it true a duck's quack won't echo?
http://www.straightdope.com/classics/a5_071.html

And finally, Frank DiBonaventuro, provides an explanation of
scientific reasons behind this rumor, on PhysLink.com:

"I'm sorry to say that it's not true about the quack of a duck. Quacks
echo as much as any other sound in nature. However, there is a way to
avoid an echo, the problem is that it depends on your distance from
the object reflecting the sound, and not the type of sound itself."

Frank's continued explanation details two distinct possibilities that
might have given rise to this observation.

PhysLink.com
Physics and Astronomy On-Line
Is it true that a duck's quack doesn't echo? If so, why? 
http://www.physlink.com/Education/AskExperts/ae482.cfm

Google search terms: duck quack echo

My research also located a short list of additional Law's of Ducks
(http://users.commkey.net/fussichen/kducks.htm). Hope you enjoy these
as well.

Best regards,

larre-ga

Well, larre, I appreciate the trouble you took to answer me, but I
knew all that already, because of course I am also a google-user.

What I was really looking for was a primary source, some
incontrovertible experiential evidence.

I can't believe there aren't duck-herds who live near walls or cliffs
who can deny or verify this factoid...

Thanks anyway, larre.

Your wrote : "What I was really looking for was a primary source, some
incontrovertible experiential evidence."
Well let me suggest to ask a primary source:
At least you can listen to a famous duck at
( http://www.sea-of-tranquility.net/sounds/nonsense/donald2.wav )
(That is similar to what I thought about your response to larre´s answer.)
You might contact the "real" duck - echo experts at
( http://disney.go.com/disneypictures/main.html )

till-ga

You can go to any one of several National Wildlife Refuges in
Southeast Arkansas or Northeast Louisiana where wintering populations
of mallards (mallards quack a LOT) can be found. In particular, those
Refuges that consist of flooded riverbottom terrain. You'll find that
thousands of mallards in one place make a lot of noise, and you can
hear the racket from an incredible distance away - because the
incessant quacking ECHOES off the surface of the water and the trees.
this has the effect of amplifying the sound the ducks make.

Some refuges where you can experience this phenomenon include:
http://upperouachita.fws.gov/index.html
http://darbonne.fws.gov/index.html
http://felsenthal.fws.gov/index.html
http://southeast.fws.gov/Overflow/index.html
http://southeast.fws.gov/whiteriver/index.html

Thank you, till, very amusing. 

And thanks also to you, slade, for your posting.

However, I checked all the sites and none of them mention echoes of
any kind, which makes me extremely suspicious. You say mallards echo,
but the US government remains stumm on the subject.

Is this duck Roswell or Grassy Mound I've uncovered here?