Hello,
I don't have a schematic to hand at the moment as I am not on my computer. I have however connected the microphone as suggested in the datasheet for a differential input. One of the mics I have tried is a Abm-703-rc Omni-directional PCB Mount Microphone 6mm.
Since posting I have found out that touching the codec only affects the output when the AGC is enabld. If I use fixed gain it has no effect.
I noticed that the noise is worse without the microphone connected, its make like a buzzing sound. So I tried powering down the Mic bias. When I do this the buzzing goes, so as a test I held a resistor to the 3.1V board supply (I'm no sure what the maximum voltage rating of the input is) however the buzzing sounds did not come back and the mic seemed loads better. I only briefly tried this as I didn't want to damage the codec.
I there a possible explanation for why the mic bias supply is so noisy?
I have two boards with slightly different layouts and they are both the same.