earXDG is a a sound theme editor for the GNOME desktop. It allows you to quickly and easily develop you own sound themes. Development has stalled, as I don't have much time to work on it, but should still be functional for most recent versions of GNOME. In my opinion, its the best way to develop custom sound themes, because it lets you avoid all the tedious manual labor of renaming the sounds. Its released under GPL, so if you want to spruce it up, have at it!