I believe that it is not possible to control the mic volume using HID or USB Audio. You will need to have a volume control on the board and control the PGA internally without PC intervention.
Thanks for your reply.
I think you are right. I tested a product of Sennheiser with the app iRig Recorder,it could not control the mic volume either. And, I tested the same product with an app named MetaRecorder, which provided by Sennheiser company, it can control the mic volume.
I believe that, the function of mic volume controlling is implement with the user-defined protocol.