Now on my 2nd iPhone 12 and still same issue of high pitched noise, crackling like blown tweeters, can't lower volume low enough either. You can hear the phone's speaker make popping sounds when people speak on the phone. High pitch noise also carries a lot of sibilance "s" on words. Tried 2 different Apple headphones and same issue - very loud high-end frequencies. Gave me a headache after just a few minutes.
I've tested and trouble shooter with apple support - even used the phone on factory setting without installing any apps that might interfere. Also did factory erase too - still no resolution. Tested audio on phone calls before and after update on version 14 and same audio issue of high pitch noise and popping crackling of speaker. Some iPhone 12s are defective either physical hardware or perhaps software release can resolve.
Very disappointed with apple releasing defective products. Makes my stomach sick because I don't have time to waste traveling to store and reinstalling and trouble shooting - ITS NOT MY JOB - ITS APPLES JOB to sell working products not defective equipment wasting our time and money and resources.