Alert sounds muted in Mac OS X Lion

Mac OS X has a separate volume setting for alert sounds. When you take a screen shot, using Command-Shift-3 (for more info, see this link) you will normally hear an alert sound like a camera shutter.

I was taking a few screen shots this morning, but I couldn't hear the alert sound. After some research, I found that Mac OS X Lion can sometimes have trouble ascertaining whether or not something is plugged into the mini-jack.

PROBLEM: alert sounds muted/don't work on iMac or Powerbook running Mac OS X Lion.

SOLUTION: plug something into the mini-jack, then unplug it. Presto! Your alert sounds should be working again.