The Mixer module takes 4 inputs and mixes them into one. The settings dialog
box lets you set a volume for each of the inputs.
The Passthrough module works like the Mixer, except it controls the volume
of 4 inputs and passes each to the corresponding 4 outputs without mixing them.
It's like 4 volume controls. Don't ask why it's here.
The XFader module takes 2 inputs and mixes them into one. The settings dialog
box only has one slider - which lets you blend the 2 inputs. You know, like
a crossfader!
The Splitter module takes 1 input and duplicates it to 4 outputs. Can be
handy.
The Panning mixer works like the Mixer, except that it has a stereo output instead of
mono and a Pan control for each input source.