The microphone on my droid mini has quit working. Calls work on bluetooth and a wired headset.
You can try to put in safe mode or even do a factory reset to confirm whether it is actually a problem with the microphone or possibly something else causing the microphone to not work.
If you have insurance, you can pay a deductible and get a replacement, which they may not have this type available anymore meaning you could get a more recent replacement model. If you have insurance on the line, of course.