Which is the only sound that does not produce an echo?

An old urban legend that has been around far too long. A duck's quack absolutely DOES echo, as does every other sound.

Sound studies have shown that since the 'quack' includes wavelengths over such a wide range, the human ear cannot distinguish the echo of the quack from the sound of the quack itself.
('Mythbusters', Discovery Channel)

An echo isn't a matter of wavelengths, it's a matter of timing of return of the sound. Any return of reflected sound that comes back after the initial sound has finished is an echo. Any sound of the right volume and in the right place WILL create an echo. However, ducks are rarely quacking in places that are conducive to the production of echoes, and tend to go on and on anyway, so it is unlikely that you will hear a duck echo. This is not to say that the quack doesn't echo.