Volume control in OS X has no effect

My volume in OS X doesn't seem to be actually controlling my volume. Not that the displayed "output volume" in the Sound preference pane doesn't change as I press volume up and volume down keys. The overlay changes, but the actual volume (the little boops) pushed through the headphones and the displayed volume in the preference pane don't change.

Shortly after taking these images, I lost all sound control via the keys. It just shows a muted overlay. The only way I could change the volume is via the sound preference pane now.

After a few minutes, I spontaneously regained control of volume via the keys. This has been happening on and off for the past week. What is happening?

Try rebooting or relaunching the process coreaudiod. This is a glitch that often results from 3rd party programs, but occasionally coreaudiod will mess up on its own.