What software version is the SX20 running?
When you mute/unmute the audio with the remote or web interface, does the microphone led change?
Have you tried both microphone inputs, and is the connector pushed all the way into the SX20?
Do you have another SX20 with a known working microphone, if so have you tried to swap the microphones to see if it's the microphone or the codec?
Have you tried updating the codec software, or factory reset (as a last resort)?