I can't say I'm a fan of Canonical's taste in sounds and wallpapers... (but I am a fan of clean methods for tweaking - as opposed to overwriting the original files)
Thankfully, as of 13.04, custom wallpapers still seem pretty easy to use.
I even negotiated the astonishingly unfriendly process of a wallpaper/settings change for Unity Greeter! (Granting lightdm X privileges as per this answer, but using the graphical dconf-editor instead of gsettings - example below)
But... The system ready drums (FluteFlute Slap?) are unchanged by assigning lightdm the custom sound theme.
The dconf-editor method works for wallpapers and the custom sound theme works for my user - so why does unity greeter not respect lightdm's sound theme?
Is system-ready.ogg played by a different user? Is unity greeter just broken in this respect? I don't want to just substitute the file in the ubuntu sound theme, but I can't see how to make the system respect the setting.
user@box:~$ sudo -i root@box:~# xhost +SI:localuser:lightdm root@box:~# su lightdm -s /bin/bash lightdm@box:/root$ dconf-editor # (Change settings various) lightdm@box:/root$ exit root@box:~# xhost -SI:localuser:lightdm root@box:~# exit