Dismiss Windows 8 Volume Box (top left corner)

I have no idea why the moderator random deleted the working solution. There is a small tool called HideVolumeOSD which can hide the overlay:

To hide the volume OSD you can select the menu item “Hide Volume OSD” or just click on the tray Icon.

With this item you can toggle between visible and hidden volume OSD. If you exit the application, the volume OSD is always restored to visible state.

You can download it here and the sourcecode can be found on github

Windows 8.1

Open the Settings Charm, click on "Change PC Settings", then Ease of Access > Other Settings, and change the length of time to show notifications for in the drop-down box.

Windows 10

Open settings and click on "Ease of access", then scroll down to the drop-down menu called "Show notifications for" and select a time.

(Picture below supplied by Semnodime)