Is this new behavior? Has it worked in the past? Any new Apps that might affect mic like Gestures Beta?
Try a Soft Reset by holding Volume Down + Power buttons until the phone vibrates (about 10 seconds), release the buttons and let the phone reboot. You don't loose data, but the system memory gets cleared. If that does not work, try a Factory Reset or Hard Reset... You'll loose data but can restore a cloud backup. Nokia Lumia 920 hard reset
Lastly it might be hardware, but unlikely since the mic works at other times.