Turning loopback audio on in Linux is a snap, sort of. Same for Windows.
Basically it’s all in knowing the right “buttons” to push.
Do this with your headphones jacked in to avoid painful feedback.
Right click on the speaker icon on the task bar to open the sound control panel and select Recording Devices. Choose the active input device (usually either a built-in microphone or one jacked in to the system) and right click for Properties. Click on the Listen tab. Check the “Listen to this device box and then Apply.
That should be all you need to do to hear the microphone input in your headphones.
While I’ve done this successfully on more than a few systems, I’m still researching how to eliminate what I consider to be unacceptable latency between my speaking and the loopback delivering the sound to my speakers. Overmodulation and ambient noise are also pretty intolerable on every system I’ve worked with. All of which means that those who have invested in expensive hardware solutions to the problem have definitely not wasted their money.