As one of my earliest Quick Tips i suggested to test your Vocoder with recorded spoken word.
In that way you don't have to hold a microphone or constantly keep an eye your audio-levels.
The audio-example that went with that post wasn't that special, but here's another short but nice demonstration, uploaded by YouTube user WorksLimited.
Video: Charlie Chaplin through a Doepfer modular A-100 Vocoder
"Charlie Chaplin speech from The Great Dictator, slowed down about 10% and run through a Doepfer Vocoder. The results compare very favorably to units like the Synton and Sennheiser (in my humble opinion)."
Uploaded by WorksLimited