Instead of using a single menu bar icon “volume control,” I have transferred the lessons of the keyboard to the GUI and placed two buttons in my menu bar: volume down and volume up. I have been using them all the time for about half a year now.

The benefits of this approach, to my knowledge and estimation, include: no waiting for a slider to appear; no nested actions; no need to read the current value; each click does not depend on the current state; Fitts’ Law muscle memory boost (the buttons are effectively infinite-height targets); discoverability compared to scrollwheelable icons.