The input signal goes from a microphone in the studio to an XLR cable which runs through the wall port into the iMac lab to the Zoom H4n input which is connected to the computer via USB.
The monitor signal goes from the computer audio out port to a 1/8 splitter. From there one side gets a set of headphones for the computer operator while the other side gets a long 1/8 to 1/8 cable which runs through the wall port to a 1/8 barrel adapter then to another set of headphones.
Here's a sketch:


