PIC32MZ: MSB of I2S data from digital microphone is always high
Your MSb is likely set because the electrical signal entering the ADC has a significant DC offset from the mid-point of the ADC. Try capturing data points and "viewing" the signal graph when there are periods of quiet and periods of high levels of sound present. If necessary capture some data and view it in excel or something similarly usable to graphically view the data.

Your microphone has a sensitivity of -26 dBFS per RMS pascal (94 dB SPL) at 1 kHz. So with an RMS sound pressure level of 94 dB at 1 kHz I would expect a digital reading of -26 dB compared to full scale. -26 dB is about 5% of full-scale so, if you are expecting the microphone to produce a big audio signal that might have a relatively small DC offset, please consider this as unlikely; your digital audio signal will be small and "sat" on top of a large digital DC offset.