How to turn off alert sounds/sound effects on Gnome from terminal?
This can be achieved by this command
dconf write /org/gnome/desktop/sound/event-sounds "false"
However, this doesn't turn off the sound volume slider effect.
To completely turn off the sound effects the closest way I've found was to live boot into a clean iso of the distro and open the System settings > Sound > Sound effects and turn these sounds off as preferred, then copy the file ~/.config/pulse/*-stream-volumes.tdb and save it. Then, to turn off the "sound effects" on an installed environment or while building a custom version of the distro do
cp saved-pulse-volumes.tdb ~/.config/pulse/*-stream-volumes.tdb