nixos/gnome3: add sound-theme-freedesktop

I've noticed a similar issue in Pantheon, without this
sound theme installed there's no system sounds.
I believe it's because the gnome theme and the pantheon
theme inherit this one.

nixos/modules/services/x11/desktop-managers/gnome3.nix

@@ -217,6 +217,12 @@ in
 
       services.xserver.updateDbusEnvironment = true;
 
+      # gnome has a custom alert theme but it still
+      # inherits from the freedesktop theme.
+      environment.systemPackages = with pkgs; [
+        sound-theme-freedesktop
+      ];
+
       # Needed for themes and backgrounds
       environment.pathsToLink = [
         "/share" # TODO: https://github.com/NixOS/nixpkgs/issues/47173