Vocoding question

[ekm]

...at least, I think it's vocoding.

What I want to do is combine spoken or death-metal vocals - i.e. no melodic qualities in the signal - with the signal from a guitar or synth, and thus have the vocals be somewhat "sung". After fiddling around with my MDA Vocoder for an hour I'm at a loss as to how I'd do this. Any suggestions?