It is definitely a software/firmware issue somewhere in the phone but I can't tell you what apps are causing the problem or what apps are unrelated. I have had this same issue since I replaced my screen with the iFixIt replacement and flashed my fingerprint scanner. So that works now, but even with these issues, I definitely do have some apps that are still working fine with the lower mic, which suggests that it is an issue with specific apps not commingling with each other or their settings/default settings/firmware/software updates are not addressing the issue or not installed/setup correctly.
For me, most (but not all) of my issues involve the pre-installed (integrated) software and/or features along with all of the Google run apps I currently have installed that call for a microphone.
Apps and features including the likes of the Recorder, Google Assistant, GBoard, or the Now Playing feature are definitely not responding to my voice, yet some apps like phone work fine.
I have tried every recommended option here (other than a factory reset) without any success. I hope there is a solution or work around shared soon, because I am not looking forward to having to hit the Reset button.
Hope I was able to give some insight that may lead to a solution and not just the same information regurgitated from the top.