The Signal Mixer is where volume and panning is set for the three Voice Panels "channels."
Channel 1/2/3 Common Controls
Channel External Level Control jacks- These are bipolar CV inputs for controlling the volume for each channel.
Channel On switches- The channel is enabled when the corresponding switch is in the down position. These are particularly useful for disabling Voice Panels when editing sounds.
Volume- Sets the volume of the corresponding Voice Panel (that's five seconds of my life I just wasted).
Pan- Sets the left/right pan position of the corresponding Voice Panel. (there's five more!)
Total Volume- Master volume setting for the entire instrument.
External Level Control jack (master)- The single jack beneath the VCA symbol is a master CV input for simultaneously controlling the volume of all three channels.