Any default microphone preference setting to change voice for anonymity?
A Mac can change its own voice, like when it's reading a text file, but not your voice.  One free software package that does what you want is MorphVox Jr. It can change a man's voice to a woman's and woman's to man.  There are examples on the link I sent. This version is free. The full version is $40.  

This link lists 5 top voice changing apps. They are Voxal Voice Changer, MorphVox, Voice Candy, Voice Changer Plus, and Voice Swap. You can read about them on the link.