Why is the microphone output level not proportional to the sound level?
Reading further into the datasheet, you are only facing a gain error, and its very linear, so you may be able to correct your gain error by using the ADC analog gain block, you can trim the signal gain by steps of +-1.5dB, page 15 of the datasheet you linked. its not a particularly great step size, but will get you within +-0.75dB of a pretty linear response.