MICBias is the voltage required for microphone to power and record signal at full levels.
What will happen if we ground MICBias, is it means we are disabling microphone.
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Yes, grounding MICBIA will cut off Mic. or at least attenuate greatly.
All electret mics need a bias since they have a common source FET at some current such that Rbias (ext) determines mic source gain and (xIP-xIN)pp max.
Shorting MICBIAs would result in poor mic sensitivity.
You must choose Rbias so expected input level optimizes SNR quality.
Make VMID well above (xIP-xIN)pp to allow for Rbias drop.
Choose VMID for MICBIAs = 2x(0.8)=1.6 x VMID.
Then VMID becomes ADC Vref i.e. maximum input which gives you a few choices of gain from AVDD1.
The datasheet must give more details.
That would be my approach....