Matt sez, "Jeroen Offerman took three months to learn to backwards-sing Stairway To Heaven and performed it *memorized*. Backwards speaking is incredibly non-intuitive; to speak in true acoustic reverse, elements like dipthongs and plosive consonants have to be reversed, and they do so in surprising ways. To truly complicate matters, add in the backwards rise and fall of the intonations of speech. When I experimented with backwards speech, this last element made my head hurt. Singing, however, effectively removes the complex pitch contours of speech, and makes things much easier. I'm not sure if Offerman had all this in mind when he set to work on his crazy project, but it very impressive for its breadth and zeal!"
(Thanks, Matt!)
Update: Andy sez, "Here's something for people to try – record yourself saying something, reverse it, record your imitation of the reversed version and reverse it again. Spooky! It's the audio version of the film trick where action is recorded backwards and the actors all have to move / lip sync backwards through the scene."