How to activate the real-time monitoring button?

Please make sure the SC1 mixer is set up as an output and input device.
If you want to monitor the audio through the mixer, you will need to press down the DIRECT MONITOR button.

How to activate the real-time listening button on the computer?

Go to "Sound Settings" - “Sound Control Panel” - “Recording” - set "fifine mixer" as default input device - disable built-in mic or other mics manually. Then double click the "fifine mixer" and choose “Listen”. Finallyclick on the box of "Listen to this device".
Make sure the computers are set to enable "Allow Microphone Access".
Check if turn on the monitoring volume (The speaker icon appears green when turned on).