Did you recently make any hardware or software changes at all, or have you connected any new headphones or speakers?
If your Mac has separate ports for input and output, the sound input port has a microphone icon. If your Mac has only one sound port, it has a headphones icon but can be used both input and output.
Does the internal microphone work with any other apps?
Ok, that solves the mystery!
Glad to hear you got it resolved.
Have a great day!