Like how the touch IC problem plagued the iPhone 6 and 6 plus the iPhone 7 and 7 plus are plagued with problems with their Audio IC. The problem manifests itself with a few different symptoms that vary a little bit here and there, I have done my best to list them all below.
The microphone works intermittently or not at all.
The speaker phone is greyed out.
Voice memos do not work.
The phone takes a long time to boot fully and there is home button response while the phone is hung on the apple logo.
The phone just shows the apple logo and boot loops
I may have missed some symptoms but these are the most common ones.The problem occurs due to dropping the phone or bending the frame. The Audio IC lies along a fault line on the motherboard where there isnt enough structural support. So when the motherboard bends or flexes or is jarred when dropped it causes broken traces under the audio IC similarly to what happens to the touch IC on the iPhone 6 plus.I have attached an image of the iPhone 7 motherboard after the audio IC has been removed and the traces have been repaired using jumper wire. This repair is particularly challenging because of the underfilled baseband chip and cpu directly on the other side. In the video below I demonstrate how the repair is done.