The way MacOSX is handling an input audio stream coming from the internal microphone is pretty buggy. Usually, you will find that your microphone is not working a minute before meeting or in the worst case when you need to say something superimportant. This bug is even more tricky because your speaker works and when you check your sound card settings in System Preferences and the audio monitor shows signal amplitude here.
The output of the ps command will look similar to this:
_coreaudiod 24243 4.4 0.1 2475428 8204 ?? Ss 10:39PM 1:01.89 /usr/sbin/coreaudiod admin 33520 0.0 0.0 2423384 232 s010 R+ 10:44AM 0:00.00 grep coreaud
You need to provide correct process ID (there will be a different number than 24243) to send the signal with the kill command.