Ifixit's teardown is for the pixel XL. It is likely different for that phone. Besides, replacement of the microphone wouldn't cut it. A microphone is an analog device, like headphones. That means, no digital signals are being sent between the mic and motherboard. So if toggling the speaker phone fixes the problem, it can't be fixed by replacing the mic.
Try downloading audio recorder from the play store. Take a recording and see if you can hear it.
If you can hear from the recording, installing a third party phone app should fix it. If you don't have it already, install your carrier's phone app.
Hope this help!