I have an iPhone 7 (normal, not plus): Since iOS 13 no mic for anything (phone calls, voice memos, Facetime, video recording, etc.). After discussing with Apple support in my country, doing all the tests they asked me to do, and from what I researched on internet, it appears that this was a manufacturing issue that became apparent after the upgrade. After upgrading to iOS 14 the camera and flashlight stopped working. Upgrading to iOS 14.0.1 solved the camera/flashlight issues but not the mic. My understanding is that the camera/flashlight was just a software issue, but since the mic problem is a hardware issue (even if caused by the software) this can't (won't) be solved with any new software upgrade.
My impression was that, like in my case, 14.0.1 solved the camera/flashlight problems for most people in this thread. But I see a few messages passing by here of people (like you) that still have the issue in 14.0.1. Did you try the new 14.1 (released yesterday)?
Before this I always thought that a software upgrade wouldn't break the hardware. (I know this is in principle possible, but I expected a company would not release a piece of software that could damage their own product.) But after this experience, I am now more cautious before upgrading.