1. In the earpiece which is used for voice memos and face time calls
2. One on the back of the phone which records audio for videos
3. On on the bottom of the phone next to the headphone jack to make calls.
The fix. Make sure those holes on the bottom of the phone are clean, if they are and you still have the same problem, replace the charger port on the bottom of the phone. The part is fairly cheep."
"It is going to be one of three things...Hardware, Software or your network.
It sounds as if you have already attempted the hardware fix, just make sure you have installed the Mic portion of your port assembly correctly and may even try to use a very small piece of cotton (Like from a Q-Tip) and pack it around the mic.
It could be a software issue, you could back up your data on Itunes, restore it and see if that helps
or...call your network provider and explain the issue. they may (or may not) be able to help depending on who you get on the line and how willing they are to try and help.
I am not talking from experience, as I have not had this issue...but did research it a bit to try and offer some help. If you do figure it out, please post back what worked for you to offer a fix for others that may have your same issue.
Assuming there are no other variables like water damage, drops, etc I would agree that a reset is the the first step if you have indeed replaced the hardware already. You can try resetting some of the other settings in General>Reset>(options) also. It may be related to the newer update if there is a software conflict with one or more apps that are installed. The iOS 11 updates were riddled with bugs for many devices.
I found a simple fix I had the plastic cover the protector on top of the phone covering the speaker make sure the hole at the top is not covered with your protective covering Once I remove the plastic and cut it down to just fit only the screen I was able to hear magnificently
