Hi John,
MicDet will not work with the way your hardware is configured. As explained in app note SLAA454, in all modes, whether detecting speakers or microphones, MicDet works by measuring the DC voltage at the MicDet Pin. This voltage will be pulled down from mic bias. In your schematic, MicDet is AC coupled so the DC voltage will not work correctly. Please take a closer look at SLAA454 to see the many examples of a microphone being detected.
Best Regards,
dave