Question about ReSpeaker 4-Mic Linear Array Kit for Raspberry Pi

Hi, I am currently trying to reuse the ADC on ReSpeaker 4-Mic Linear Array as a general-purpose ADC to capture other audio-like signals like vibration signals. The reason for doing that is I think this is the easiest and cheapest way to sample signals and do frequency analysis up to 20KHz on raspberry pi. I was wondering if there are some caveats of doing that. Currently, I found several problems:
(1) I cannot get the exact voltage measurement. The signal seems to be scaled. Maybe it is A-weighted?
(2) Sometimes the signal is too large so the ADC result will be MAX. My current solution is to use ALSAMIXER to adjust the signal to make sure it is in the right range.
Any other comments of doing that?