If MirrorOp was installed before the latest macOS update, upgrading to the latest MirrorOp version will work and you will be able to share video and sound as intended.

But when installing MirrorOp for the first time, from MacOS version High Sierra, onwards then you will be able to share the video but not by default with the corresponding sound.

• On macOS High Sierra: no sound is shared.
  
  
• On macOS Mojave: the sound captured by the microphone is captured and shared…

When pressing the unmute icon on the MirrorOp application you will see a popup to install SoundFlower as well [Yes][No].  

When trying to install it, by pressing the Yes button, the installation of that sound software driver will be blocked by macOS.

In recent updates, macOS has increased the security level. If a user installs a driver which is not from an identified developer, the system will only pop up a message showing "XXXXX cannot be opened because it is not downloaded from App Store." He still needs to enter System Preference manually to allow this installation.