Are the PMS SCW/SDW wires connected to the Wii properly? If the PMS can't communicate with the Wii it can't pass along the volume wheel state.
Can you check that the volume wheel is getting 1.8v and has a good GND connection? Also worth checking the output voltage of the middle leg. Might be a...
The RVL-AMP doesn't have an ADC to handle analog volume control, you gotta use the spare ADC on the PMS instead
If you have the older revision PMS2 or older lite model you have to use one of the JTAG pads
If you have one of the new revisions there is a dedicated pad for it
The top row look to be analog audio inputs. RIN would be right audio in and LIN would be left audio in.
Second row I think are output lines for speakers. I am guessing VORN is Right Negative and VORP is Right Positive, which would make VOLP Left Positive and VOLN Left Negative.
If there is...