Hide or remove GNOME pop-up for sound volume level indicator
When I scroll the mouse over the volume control a volume level indicator is shown in the center of screen.
How can I remove this pop-up sound volume level indicator?
It's a built-in "feature" of GNOME shell, so it's probably not easily removable by default. You may use a GNOME extension called "Better OSD" as a workaround. This allows you to reposition and resize OSD pop-ups like the volume pop-up you have mentioned.
It also has a "Hide Delay" option, setting it to "0" should make the pop-up invisible.