/usr/share/gdm/autostart/LoginWindow/libcanberra-ready-sound.desktop is in gnome-session-canberra 0.30-0ubuntu3.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 | [Desktop Entry]
Type=Application
Name=GNOME System Ready Sound
Comment=Plays a sound whenever your system is ready for login
Exec=/usr/bin/canberra-gtk-play --id="system-ready" --description="GNOME System Ready"
OnlyShowIn=GNOME;
AutostartCondition=GSettings org.gnome.desktop.sound event-sounds
X-GNOME-Autostart-Phase=Application
X-GNOME-Provides=login-sound
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