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.
-2015 F30 320i, Imperial/Oyster, 8AT: August 2014 ED
-2012 E92 M3 Jerez Black/Rust Brown, M-DCT: September 2011 ED
-2008 E90 M3 Melbourne Red/Bamboo, 6MT: Gone
-2007 MK5 GTI United Grey/Grey, DSG: Gone
-2004 E46 M3 Carbon Black/Cinnamon, 6MT: Gone