Here's a post I just dropped in the main thread... it's not totally fixed, at least from my testing:
Not sure if this was mentioned, but I called a friend that I talk with often on CarPlay, specifically to test after the 16.0.3 update, she said the sound quality is improved, but the volume is only a little better. By that she said she normally put me at half volume on her iPhone (using FaceTime Audio btw) when I was on iPhone 11 Pro Max, and once on iPhone 14 Pro Max of course it was almost a whisper as we all know, but NOW after 16.0.3 she said it's better than it was, but she's got me up to 75-80% to hear me the same way. I could hear her fine at the same car volume I run. I trust her assessment as she works with video and audio editing on her MacBook Pro in her home studio, so she has half a clue. So it's not a complete fix on the mic side. I'm posting this to Apple next.