It might be worth mentioning, I have just checked, that neither http://www.mate-look.org/ nor http://www.gnome-look.org/ have sound themes per se, but rather assorted individual sounds.
I stand corrected, my apologies: on both sites, searching for "sound themes" and then filtering by category "System Sounds" will show a plethora of sound themes.
It might be worth mentioning, I have just checked, that neither http://www.mate-look.org/ nor http://www.gnome-look.org/ have sound themes per se, but rather assorted individual sounds.