Hi Frank, tmo. it is not possible to change the default setting of the mic in the OS. A simple 'workaround' is: just set your own microphone setting and store it in an Registration Memory. Once you start up your keyboard one simple Memory push is enought to set it to your wanted volume etc.
Be aware that using the Freeze- and/or Reg.Memory (the check marks on or off) influence the microphone settings!