Why is the Microphones page grayed out?

The microphones page is only active prior to the initial MIDI file link. Due to the way Realize generates audio output, the microphone configuration must be decided upon in advance and no changes are allowed once the MIDI file has been linked.

If you are looking to have independent mix capability over the mics, we suggest that you create a multiple instance project. One instance of Realize for each microphone set, with the level set at zero for all mics except the one intended to be used with that instance. Then you can link the same midi file to all instances and each instance will output only its microphone set. Then you can use your DAW faders to control the mix, including any automation capabilities native to your DAW.