[AC Question]

The sound is all the way up.

[B. Diddy]

Welcome to Android Central! The volume setting doesn't affect the microphone. Are you saying that the microphone doesn't pick up sound any more? First, check to make sure the mic opening isn't blocked or clogged with any debris. If it's clear, then does this persist after a reboot? Or how about Safe Mode? On most Android devices, while powered on, press and hold Power until the Power Off menu appears. Press and hold the Power Off selection until the Safe Mode prompt appears. Tap OK.

If the problem persists in Safe Mode, then the microphone has probably failed. Bring it to a repair shop, or contact the manufacturer if it's still under warranty.