An electrolanrynx is a medical device that helps people speak who have had their voice boxes removed due to throat cancer. It typically produces a monotone or "robotic" voice.
A Theremin is an electronic musical instrument that uses two antennas to control pitch and volume. It was often used to
create eerie sounds for old sci-fi films, but a skilled performer can also reproduce many of the sounds of a human voice, except for consonants.
These two devices could be combined together to create a more natural-sounding voice, and even allow the user to sing. Making the device wearable or hands-free might be a challenge, so it might not be practical for everyday use.
Electrolanrynxes have been produced that have some adjustability in terms of volume or pitch, but these are limited in how well they can reproduce the full range of human speech.