The only time Morn ever actually speaks during any scene that made it into the final cut (according to Mark Allen Shepherd, the actor who played Morn) was in the background of a scene being shot
The circumstances surrounding my first day on the show were so unusual. When they shot the bar scene in Quark’s where Morn is telling the funniest joke in the universe, they told me I could go ahead and tell my joke. And when Sisko gets off of the turbolift and just as he walks towards Quark’s, if you listen carefully to the background sound you might actually hear Morn saying “Finalism finger fink. Obligatory quotient yokefellow, coconut kachina cosmological argument. Bank swallow fish story, inculpate minuteman. Stress certifier in lecithin. Hard-hearted dill, divine minded domineer, mind reader sextuplet, garden fly honey suckle garbage. Palter rimfire, green peace. Change is the ultimate solution. Twenty-fourth century humor. You may not get it right away.”
In the book The Making of Star Trek: Deep Space Nine, the first chapter of the book is “The Funniest Joke in the Universe” and it is about exactly that and how I came to be on the show. That was the only time I got to say anything while the cameras were rolling, but it´s not the only time that they considered Morn for a speaking part. There have been numerous scripts where they originally had Morn speaking. One such script had to do with the alternate universe. Unfortunately, they always got written out.