It confounds me as to why Apple will provide an easy way to get a reference a standard system sound (via -soundNamed:) but doesn’t provide any easy way to get a list of available system sounds. Well, as you might of guessed, I’ve created a NSSound category that adds this functionality. The code is adapted from this cocoa-dev post. You can download the source files (BSD license) here.
On a side note I’ve updated the source code page. The underlying code is now alot cleaner (and renders correctly on IE!) and it uses some of those fancy AJAX transitions.