All one has to do is make a broken alias, and Mac OS X will flash the screen when it would have played the sound. The flash is the same as in Universal Access -- I imagine this inadvertent feature is the result of defensive programming on Apple's part.
A walkthru:
- Open a new Finder window, pointing to ~/Library/Sounds (create the Sounds folder if necessary).
- Open another Finder window, pointing to /System/Library/Sounds.
- Select Sosumi.aiff from the System folder and drag into the Sounds folder window. This will automatically make a copy, as you don't have permission to move items from the System folder.
- Close the /System/Library/Sounds window.
- In the ~/Library/Sounds window, select Sosumi.aiff, then choose Make Alias from the File menu. The default name for the alias will be Sosumi.aiff alias, and it will be selected and ready to rename. So, rename it to something appropriate like flash_screen.aiff or flash.aiff.
- Select the Sosumi.aiff file in ~/Library/Sounds and drag it into the trash.
- Empty the trash.
- Quit System Preferences if it's running, then start it and go to the Sound -> Sound Effects tab. You should now see Flash as a custom alert sound, and as a new mail sound in Mail.app, and other places).

