Windows 10 - Put Clock and Audio Icon on All Monitors

As of Windows 10 version 1803 this still seems not possible. The clock will now by default display on all monitors, but the notification icons (such as the volume control mentioned in the question) only show on your "Main Display".

The only options currently available are (a) switching your "Main Display", or (b) using third party software.

Although I have been unable to show the system tray on all monitors, you are able to move the main taskbar and system tray to another monitor. You must unlock the taskbar and then drag it over to the monitor you want it displayed on.