Answer

OSD(On Screen Display) menu volume setting is available only when you use the All-In-One as a monitor. Please plug the HDMI cable into the HDMI IN port if you want to use the All-In-One as a monitor.
If you're using All-In-One as a desktop, please use the volume adjusting bar at the bottom right part of the windows taskbar.

Models

Modern AM241T 11M, Modern AM241TP 11M, Modern AM241P 11M, Modern AM271 11M, Modern AM241 11M, Modern AM271P 11M