First of all i want to say THANKS for customers who has raised this issue, i know im not ALONE with this issue.
Same issue for me, this muffled mic happens after i reset AirPods Pro,
Then nothing helps:
Changing mic for left or right
Reset Airpods Pro,
Connecting from bluetooth,
Connecting from notification window whats informing you about fonded airpods Pro --- nothing work, just nothing, muffled mic, broken Active noise cancelation.
For me it like headphones which using same mic what working for noise cancelation and for talk/voice or video calls/recording voice mic sound from mic very bad i can't believe apple can make products like this, without pre testing and then asking customers to send their airpods pro back to apple for testing, i was changing airpods 6 times and i will stop when AirPods Pro will working like they need to work, Apple released 2 firmware updates and both are very bad, fix fix fix and very fast, a lot of customers are not happy with Pods Pro
2 month or more with ANC issue - https://discussions.apple.com/thread/250887615
2 month or more with Muffled mic on voice/video calls and voice recordings - https://discussions.apple.com/thread/250813242
Waiting for update, in this time i'll just change them every 10 days.
Fix this in next update or feel free with refund for customers :D