Rubber Microphone Cover Availibility

Are you sure it comes with a rubber enclosure that goes over the bottom microphone on the charging port flex? Last time I replaced the charger port I'm pretty much sure it justs sits aligned to the microphone hole with a plastic thing that is screwed to the housing and makes sure the microphone is in contact with no gap to the microphone hole.

Check the bottom microphone filter to see if it's transparent or not. If it isn't then it's most likely blocked and needs to be cleaned.

If microphone does not work with speaker phone mode activated during calls then that is the top microphone which is part of the front camera flex ribbon cable assembly that is having issues. Same deal there, might need cleaning.

To test microphones, tap near the microphone area and see if the microphone registers it or you can hear the tapping noise during call / voice memo. If you can hear the tap but they can't hear you then the microphone filters / meshes most likely need to be cleaned.

Usually they don't go bad out of nowhere they most of the time have their vents clogged / blocked by dust / dirt etc.