This app was mentioned in 2 comments, with an average of 1.00 upvote
Is it possible that these users are experiencing some form of this https://plus.google.com/u/0/+ArtemRussakovskii/posts/DztjhACgc8j, considering their mics do work sometimes?
In other words, some app muted the mic, which, as I pointed out in the G+ post, is, shockingly, an operation that sticks even through reboots. Try this app to unmute and see if the mic works again: https://play.google.com/store/apps/details?id=com.bettertomorrowapps.microphoneblockfree (toggle it a few times just in case and leave in unmuted mode).