Help us change this! There are many web sites like art.gnome.org which provide a large selection of graphical themes for Gtk+, Metacity, icon sets and so on. We want to see a similarly large selection of sound themes available! And we'd like you to contribute to this!
Please note that only subset of the sounds listed in the Sound Naming Specification is currently hooked up properly -- i.e. generated when "input feedback" is enabled or triggered by applications. Nonetheless it makes sense to include them in your theme, because eventually they will be hooked up.
When you put a theme together, make sure that you only select sounds that have a sensible Free Software license -- or if you have produced them yourself you pick a good license yourself. GPLv2+, LGPLv2+, CC-BY-SA 3.0 and CC-BY 3.0 are good choices.
Not everyone is as lucky as Richard Hughes and has a mom who is practically an endless source of special effect sounds. If your mom sucks then don't despair! The OLPC team has compiled a huge set of Free sounds that is waiting to be made an XDG sound theme. I am eagerly looking forward to your sound themes that make use of "The Berklee Sampling Archive - Volume 13 - synthesizer - fx (126 samples) spaceships, lasers, explosions, machineguns, glisses" to start a war in space each time you click a button on your screen!
 Free as in free desktops that is.
 OK, to be honest I am not actually that eagerly looking forward to that. Spacewar-at-your-fingertips is pretty lame in comparison to a theme called "Richard's Mom".
 You have no idea what all those Hughsie's-Mom-jokes are about? Then listen to the sound files that are shipped with gnome-power-manager!