You need to determine if the problem is modular in nature or not. There are 3 microphones in the iPhone.
* The bottom microphone is for Voice Memo and phone calls. The other microphones serve as error cancellation
- The (top) front microphone is used for FaceTime calls, Siri and selfie videos with the front camera (FCAM). The top back microphone serves as error cancellation
- The (top) back microphone is used for videos with the rear camera (RCAM). The top front microphone serves as error cancellation
If the issue affects more than one microphone, then it could be the Audio Codec IC on the logic board. The iPhone 7 series of devices have proven to be very unreliable, suffering from flex-based issues similar to the iPhone 6 Plus (Touch Disease). On the iPhone 7, the fault line runs along the top of the SIM card reader and affects the Baseband CPU and Audio IC. Any short copper traces running perpendicular to the fault line while connecting to micro-BGA pads, is subject to failing.
On the Audio IC, this particularly affects the C12 pad/trace although there are 4 pads/traces along the fault line. Of those four, the C12 pad (I2S_AP_TO_CODEC_MCLK) is the most critical as it is a communication protocol line between the CPU and the Audio IC.
Here are the typical symptoms, as collated by the repair community:
- Voice Memos app / Loudspeaker on call - greyed out
- Siri doesn’t hear you / Caller can’t hear you
- Most other Audio related issues that aren’t solved by modular replacements
- Long boot times (3-5 minutes)
- iTunes detects the phone but stuck on the Apple Logo
This is a repairable problem but it requires a micro-soldering repair.