Digital Microphone : I2S data output
I would bet that your I2S receiver is incorrectly configured, I2S has a rather weird one clock offset compared to the other common formats, and if you get this wrong you will always see a 1 in the MSB of the recovered sample which causes everything to appear negative if interpreted as a signed value. 

Check your programming of the port against the timing diagrams for I2S, my bet is you are set up for right justified not I2S. 

Getting the sign extension right for this can also be 'interesting'.

The units are either pressure or velocity (depending on which type of microphone element) relative to some slowly filtered baseline pressure. Scale depends on sensitivity.