How to bring up drumroll sound at login screen in 20.04

Historically the system-ready sound was contained in ubuntu-sounds package which is used by unity-greeter.

To get drum sounds you have to install LightDM and configure it to use Unity login greeter.
Analysis of the logs gives a clue that we need to install PolicyKit service.
So the complete reproducible command is below:

sudo apt-get install lightdm unity-greeter ubuntu-sounds policykit-1 --no-install-recommends
# select lightdm in ncurses window

Then reboot and enjoy.

For Ubuntu MATE 20.04 LTS the setup is different. It uses slick-greeter.
To configure it we need to create special configuration file and install ubuntu-sounds package and specify path to the sound file:

sudo apt-get install ubuntu-sounds

cat <<EOF | sudo tee /etc/lightdm/slick-greeter.conf
[Greeter]
play-ready-sound=/usr/share/sounds/ubuntu/stereo/system-ready.ogg
EOF

Then reboot and enjoy.