For changing voice, FMOD does not really have any easy built-in parameters or tools to implement the function you described.
I would suggest checking out our record example at FMOD Studio API\api\core\examples\record.cpp. This example shows recording from the microphone and applying a DSP effect to it with live playback. You can swap the DSP effect out for any other effect you desire.
For more information, I will link a similar post here: