After a considerate amount of banging my head against walls, I finally managed to add a microphone (i.e. audio IN stream)
to the AudioClass code CubeMX generates.
I'm attaching the code as a patch to what CubeMX generated for me (YMMV). This is tested only to the extent that ramp data appears on the host side when recording. I wanted to share while it wasn't application specific.
the patch is licensed under a standard "Free Beer, v42" license, enjoy.