Nothing shows in the midi monitor. However, when I take the muxshield of the arduino and then connect the wires as one would in a situation without mux, the midi monitor shows exactly what you would expect. In the case of the small midicontroller
So there must be a difference. According to the schematic there is nothing that the shield is doing to the normal behavior. It is hard to see from your pictures but the ground wire looks like it is a pin out. Try using the ground on the other side. Alternatively there could be a miss soldered joint or bad socket on the extension sockets bringing the signals from the arduino to the top of the shield.