The Increase System Sound Volume and Decrease System Sound Volume actions increase or decrease the system volume by one unit.
To mute or unmute sound use the Toggle System Sound Mute action.
When you use these actions the system volume overlay briefly appears on the screen.
If the System Preferences Sound setting Play feedback when volume is changed is checked then you will hear a beep when the volume changes.
To set the system volume to a specific value use the Set System Output Volume action. Set the slider to the desired level. The action tells you the specific percentage level of the slider position. No overlay appears on screen with this action and there is no sound feedback.