KTB control signals information

While I wait for my board to be delivered. I was wondering if details could be shared on a few things:

  1. What’s the function of the “VOLUME” signal on J3(pin-13)? Can I have electrical specs?
  2. Is it possible to have the electrical spec on the “MUTE” signal on J3(pin-16)? My assumption is that it is a 3V3 active high or low signal.
  3. How can I change the input from USB to SPDIF and vice-versa?
  4. What’s the function, if any, of “XMOSIOx” lines on J3(pins-9,10,11)?