If you are plugging a mic dirrectly into the C, chances are you will need to add gain to the input (FX edit menu from memory) - this creates a fair bit of noise and isn't the best way to add a mic.
The best way to use a mic with the C is to use a preamp before the input.
Honestly though, in my opinion you will get a nicer result using the Korg's vocoda than the C. Although the C's vocoda is quite editable, it just doesn't have the clarity - real pitty.
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