MICBias Ground in audio codec
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....