Conor Mac Aoidh's Blog: OS X: Audio Alerts for PHP Errors
Kae posted today about tackling this problem under Linux, but I've found that it's quite a different task under OS X.
It also uses some additional Perl modules (like Date::Calc and Date::Manip) to set up swatch. From there it's just a matter of adding a few lines to a configuration file to watch for the errors and sound the "bell". A few bash scripts will need to be added to correctly start the service too.