you can monitor the audio output using ProLevel/ProPhase with the BlackHole virtual audio driver.
BlackHole: Virtual Audio Driver
BlackHole is a modern MacOS virtual audio driver that allows applications to pass audio to other applications with zero additional latency.
https://github.com/ExistentialAudio/BlackHole
How to setup a multi-output device:
https://github.com/ExistentialAudio/Bla ... ut-devices
NOTE: For a multi-output device, due to issues with macOS, the Build-in Output must be enabled and listed as the top device in the Multi-Output.
Audio MIDI Setup app:
ProLevel/ProPhase app:
Sound pref pane in System Preferences app: