Instructables has a new project up that looks at how to build an analog vocoder, the Morphatron:
I designed the circuit from scratch and I am making it available to all so anyone interested can build their own. I have called it the Morphatron. I will not say it is a piece of cake, but I have done most of the hard work for you, and either way it is very much worth it, because this machine can acomplish amazing things.
The tutorial includes schematics and parts lists, but is a more advanced synth DIY project.
Here’s a demo of the vocoder in action: