Homegrown Sounds released Input Monitor, a free Mac application that simply passes a selected input pair to an output acting as software monitoring. If you find yourself needing to open a DAW to listen to audio plugged into your soundcard inputs, the ‘Input Monitor’ is a lightweight alternative.
When opened it automatically begins monitoring the system selected input to the system selected output.
It was created to solve the issue with direct monitoring with a Steinberg UR22 mkii which is always dual mono.
More info and free download here: Homegrown Sounds | Input Monitor