The real reason why Six has a different voice in every season.
It's likely Chode (or Six herself) had been tinkering with her vocal components (similar to changing settings on a TV).- Alternatively, Six has to change her voice box repeatedly because it keeps getting worn down by frequent sexual activity. Considering Chode's libido, it would be necessary maintenance.
Six possesses a whole collection of spare parts that she can install into her body.
Shown in the "God is Our Pilot" and "Skankenstein", Six's body parts are removable. She probably has a bunch of special body mods when she sleeps with Chode. Considering Chode's history of being subjected to Prison Rape, and his Closet Gay tendencies, Six might even have a phallus stored in Chode's room. Chode is secretly Bisexual.
Openly, he's turned off from meeting men and transwomen, and treats Gus terribly. When he's alone with Six, he's far more comfortable with indulging his fetish with her, especially when she's equipped with a penis. Chode could have removed Six's advanced programming, but doesn't because he secretly prefers her being smart.
Chode truly appreciates Six because of her intelligence and independence.