My external microphone stopped working after I successfully repaired a failed telephone ear speaker. I'm not sure if the mic failure was related to my speaker repair or just a coincidence. The mic was not disturbed during the speaker repair.
The repair went well. All parts on the ribbon cable supporting the mic (earphone jack, volume buttons, silence switch) work properly.
The silence switch is held onto a metal mounting plate with adhesive AND two plastic posts that are melted to form rivet heads. Removing the old ribbon cable and integral silence switch involves breaking the adhesive bond and breaking the plastic rivets. After adhering the silence switch to the metal mount, I used a soldering iron to melt the (new) plastic pins, mechanically securing the switch.
When I powered the iPhone 4S on for the first time following the repair, the cell phone would not transmit or receive. I got the cell phone working using the solutions found here - iPhone 4S keeps searching after screen replacement It would be a good idea to reference these instructions any time the battery is disconnected in (at least) a 4S.