Hello,
I found out that when you normal use the phone, that it uses 1 microphone at the bottom, near the charging connector.
When you have the phone in speaker mode than it uses all 3 microphones, the one at the bottom, at the top and the one next to the camera lens.
My experimenting shows that if you speak loud when using the phone in normal mode the sound is OK. When you start to speak at normal level or at lower level the microphone at the bottom starts to "clip" the audio to a very low level, if you rase your voce again the sound is good again.
After lot's of contact with apple and two complete new clean installations with Itunes the problem was still there.
I told Apple I want my money back or a new Phone. They only wanted to repair my new Iphone 8.
I went back to the store where I bought my phone and changed my phone for a new one.
After I got home I did a clean install as a new phone and inserted my SIM card.
I phoned my wifes phone and put it to my ear while I started to speak soft in my new phone.
All sounded OK, no problem with low voice level. I switched over to speaker mode on the new Iphone 8 and it sounded the same OK, I switched back to normal mode and now the problem was there again, at low voice level the Iphone 8 microphone cut's the audio level to a even lower level. I switched the option for reducing audio noise OFF in settings but no difference.
So this is my second brand new Iphone 8 with the same problem.
Next week I get a brand new SIM card from my provider to check if it has anything to do with the modulation from the lower microphone to the GSM network.
When I have new experiment results I will post them here.
Gr