When you increase or decrease the volume on Widows 10, or mute/unmute your speakers, an on screen display alerts you to the change in volume. It appears at the top left of the screen and stays on the screen for a minimum of five seconds. If you want to disable the annoying display, read on https://www.addictivetips.com/windows-tips/how-to-disable-the-volume-on-screen-display-in-windows-10/