One of the first things I do when setting up a Mac is change the following setting, which I expect very few people know about. It’s great if you like to keep a machine muted, or listen to music, or leave headphones plugged in when you’re not wearing them.
In System Preferences → Universal Access, go to the Hearing pane and switch on “Flash the screen when an alert sound occurs”. That’s it. Personally I also then turn down the alert volume, and also switch off the volume change notification sounds, which means I can listen to music without the computer interrupting, or leave headphones off and still find out that my bash prompt is “beeping” at me.
(I did mention this previously, but thought it worth being a bit more informative about how to do it.)