Do you mean just alert sounds, or interface sounds as well? If you already have aiff files that you want to add just for alerts, you can drop them into your User/Library/Sound folder; if you want to bring back Classic sound effects ("Appearance Sounds") you can get Xounds from Unsanity. Also, if you download BBX Mercury from Max Themes, it comes with a bunch of really great alert sounds that you can install, even if you choose not to use the theme itself.