The only microphone adjustment in this particular OEM DIRAC system was done at development, according to BMW. There is no active microphone adjustment in this system, only speed- and bass-equalizations.
One feature that really changes the audio response of this system is the speed-adjustment (GAL). Anything above 1-square will change the EQ curve that you set above 40mph or so.
This GAL implementation is the most aggressive that I have seen in all BMW systems.