Does anyone if you can add a sound here to use for the default alert tone?
This folder seems to have the default sounds in it.
/usr/share/sounds/gnome/default/alerts
There's a file called.
/usr/share/gnome-control-center/sounds/gnome-sounds-default.xml
Add a new sound section to this file and change the filename and name items to your new file.
<sound ...>
...
<name xml:lang="en_GB">Sonar-jg</name>
<filename>/usr/share/sounds/gnome/default/alerts/sonar-jg.ogg</filename>
No need to restart, just reopen the settings app and your new sound will be listed.