by Default in Ubuntu 18.04,
gsettings for the schema org.gnome.desktop.sound and the key event-sounds is true
pratap@i7:~$ gsettings get org.gnome.desktop.sound event-sounds
true
pratap@i7:~$
making the key value to false will stop all the event-sounds..
to stop only the battery-full event-sound..
check the sound theme-name
pratap@i7:~$ gsettings get org.gnome.desktop.sound theme-name
'ubuntu'
pratap@i7:~$
List and find if it has battery-full sound file..
pratap@i7:~$ ls /usr/share/sounds/ubuntu/stereo/
bell.ogg dialog-information.ogg phone-outgoing-calling.ogg
button-pressed.ogg dialog-question.ogg service-login.ogg
button-toggle-off.ogg dialog-warning.ogg service-logout.ogg
button-toggle-on.ogg message-new-instant.ogg system-ready.ogg
desktop-login.ogg message.ogg window-slide.ogg
desktop-logout.ogg phone-incoming-call.ogg
dialog-error.ogg phone-outgoing-busy.ogg
pratap@i7:~$
Example Sound theme that have battery-full sound file..
pratap@i7:~$ gsettings set org.gnome.desktop.sound theme-name Smooth
pratap@i7:~$ ls /usr/share/sounds/Smooth/
documentation index.theme stereo
pratap@i7:~$ ls /usr/share/sounds/Smooth/stereo
audio-channel-front-center.oga menu-popdown.oga
audio-channel-front-left.oga menu-popup.oga
audio-channel-front-right.oga menu-replace.oga
audio-channel-rear-center.oga _message-new-email.oga
audio-channel-rear-left.oga message-new-email.oga
audio-channel-rear-right.oga message-new-instant.oga
audio-channel-side-left.oga message-sent-email.oga
audio-channel-side-right.oga message-sent-instant.oga
audio-volume-change.oga network-connectivity-error.oga
battery-caution.oga network-connectivity-established.oga
battery-full.oga network-connectivity-lost.oga
battery-low.oga notebook-tab-changed.oga
button-pressed.oga phone-incoming-call.oga
button-released.oga phone-outgoing-busy.oga
button-toggle-off.oga phone-outgoing-calling.oga
button-toggle-on.oga power-plug.oga
count-down.oga power-unplug-battery-low.oga
__desktop-login.oga power-unplug.oga
_desktop-login.oga system-ready.ogg
desktop-login.oga trash-empty.oga
desktop-logout.oga window-attention-active.oga
desktop-switch-left.oga window-attention-inactive.oga
desktop-switch-right.oga window-close.oga
device-added.oga window-maximized.oga
device-removed.oga window-minimized.oga
dialog-error.oga window-new.oga
dialog-information.oga window-slide.oga
dialog-question.oga window-switch.oga
dialog-warning.oga window-unmaximized.oga
link-pressed.oga _window-unminimized.oga
pratap@i7:~$
Rename the file battery-full.oga with below command..
sudo mv /usr/share/sounds/Smooth/stereo/battery-full.oga /usr/share/sounds/Smooth/stereo/battery-full.oga.bak
Thats it.. to make a event sound.. it cant find battery-full.oga as it has been renamed as battery-full.oga.bak