update on what i've tried so far:
- reboot & hard-reset
- reset network settings and all other settings too
- restore iPhone from iTunes
- switch off BT and reboot
- switch off WiFi call option and reboot
none of it worked.
i'm not surprised that not everybody has the same problems. even though they are all iPhones 7, i doubt that all devices are 100% technically identical. (from my own experience in other product development this is rather unlikely).
i noticed yesterday, that for some funny reason the green dot in the upper right corner (showing that the mic is working) suddenly showed up in the home screen once, and even in the locked screen once.
i have no explanation why.
today i tried to downgrade to iOS 13.7
so i got myself an older iPhone backup from the time-machine, as well as the matching IPSW (firmware).
but that won't work either. Apple does not allow anymore to sign this OS version.
if at least they would allow us to downgrade again, until they fix the bugs in the iOS 14.
if we could downgrade and all of us still had the trouble with the mics, then they could convince us that it's a hardware related issue. but if they don't even allow us to find that out, this is a pretty unfair behaviour.
it's possible that they messed up. probably they haven't.
but if they had, they would be aware of it, and they should at least give us a solution as "easy" and quick as downgrading!