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.

Option 1

You cannot achieve this directly through System Preferences » Sound » Input. If you're looking for fleⅺbility, low cost (or free), maⅺmum variation, natural sound, and the ability to carry on a real-time conversation, consider using a digital audio workstation (DAW), with a combination of:

1. Soundflower (free, routes audio)
2. LineIn | SoundSource 3: (free, combines mic and audio sources)
3. Audio production software/DAW
   • Adobe Audition (commercial)
   • Apple GarageBand (free)
   • Audacity (free & open-source)
   • Audio Hijack (commercial, does not require items 1. and 2.)
4. Change voice. Some of the best effects to start with are:
   • Raise pitch
   • Stretch and pitch
   • Bend pitch
   • Distortion
   • Doppler Shift
   • Reverb
   • Modulation
   • Delay
   • Echo

Option 2

If you do not need to carry on a realtime conversation, you could write what you have to say and have Acrobat narrate it.

Save your script as a PDF. Setup QuickTime to record audio from your computer. Depending on your set up, you may need to install Soundflower to route your audio correctly. Start recording in QuickTime. Open Adobe Acrobat and press ⇧+⌘+Y or click View » Read Out Loud » Activate Read Out Loud.