How do I adjust the volume of the microphone on my AirPods Pro?

AFAIK, there is no adjustment. It is either on or off. See the 'Set Microphone to left, right, or automatic' section in:

Change the settings of your AirPods and AirPods Pro - Apple Support

AirPodsPro66 Said:

"How do I adjust the volume of the microphone on my AirPods Pro?"

-------

Leave Apple Your Feedback:

Do Your Part: To get your word out to Apple on this, you need to provide Apple feedback.  Apple won't get back to you directly, but the more feedback they receive on this, the more they will know what is going on, and what needs to be fixed.

1. Go Here: Feedback - AirPods - Apple - Apple
2. Select: "Feature Request" for the "Feedback Type"
3. Comment: why you think it should be a feature of AirPods, and how you think it can be of great use. Include the URL to this post
4. Proceed from there as necessary