Well, now I've inadvertently disabled my desktop login sound entirely. I went to Control Center -> Startup Applications Preferences -> GNOME Login Sound and edited that to play /usr/share/sounds/stereo/desktop-login.ogg. When I rebooted, there was no sound at boot-up. So I did it again, this time selecting /usr/share/sounds/linuxmint-login.wav, but with the same results (no login sound). I stupidly forgot to write down what was there by default, so I can't get that back.
I also tried selecting one of the default themes (Mint-X or Mint-X-Metal) but that only changed appearance, and not the sounds.
Can anyone point me (us) to the default sound file at login? Or even better, set up all three gdm event sounds (desktop ready, login, logout)!

UPDATE: I restored my login sound according to
this post. Thanks to kmb42vt!
The default is /usr/share/sounds/LinuxMint/stereo/desktop-login.ogg. I still don't know how to configure the greeter or shutdown sounds.