I took an excerpt from Star Wars Episode V: The Empire Strikes back, the famous scene where Darth Vader says "I am your father."  I took as my background sound the first part of their lightsaber fight leading up to the dialogue (which, conveniently, is about 22.5 seconds long), and then I used their dialogue as my speech.  My files are:

lightsaber.wav  (the spectrally rich battle)
father.wav   (the dialogue, followed by Luke screaming)
ctralie4.wav  (the final product using fft.c and vocod.c)

Darth vader's voice, which sounds like it was already made with a vocoder, sounds even more mechanical here.  It's nuts to hear them "talking" through their lighsaber battle