How to hide FaceTime audio macOS High Sierra Version 10.13
This seems like a bug.

Also other people had this kind of question too: Apple discussion

I have found three temporary workarounds (the last one is the best). All of them force quit the process FaceTimeNotificationCenterService:


Open Activity Monitor and search for FaceTime. Then select FaceTimeNotificationCenterService, press the red circled button and choose Force Quit.


Open Terminal and type killall -kill FaceTimeNotificationCenterService then press return (⮐)

With this command you essentially Force Quit the process. (For more information type man killall; -kill sends the signal kill instead of the default signal term to the process)
For a more permanent solution you start Automator make a new Application. Here you go to Utilities and drag Run Shell Script to your workflow. Write killall -kill FaceTimeNotificationCenterService in the text field and you can save the document.



As a shortcut you can place this Application in your dock so it needs only single click to activate.