If you need to appear in a Google Hangout for whatever the reason, you may have noticed that the instructions on how to get the audio and video plugin to function in High Sierra's version of Safari are wrong. Here's how to make it work.
The first, and only, thing Google got right in the entire procedure is to download the plugin installer, and run it. The plugin installer runs without complaint, and will ask for your user credentials to install.
However, just installing the plugin isn't enough, and Google's suggestions about simply allowing the plugin to execute aren't complete. Until you tell Safari to allow the browser to use the camera and microphone, it won't even pop up the dialog box to ask if you want to use them or not.
In the Safari menu, choose Preferences. Then Select Websites, and click on Camera.
Toggle the Configured Websites for the Camera to Allow
Repeat the process for Microphone.
And, you're still not done if you have a USB microphone, camera, or headset. Click on the gear icon on the top of the open browser window. Select from the pull-down which option you want -- as it defaults to the internal microphone and speakers, regardless of your system setting.