How to add custom alert sounds to Gnome (Ubuntu 18.04)

I’m currently doing my yearly-ish reinstall of my OS. This time doing a fresh install of just basic Ubuntu with gnome, having previously used Ubuntu Mate for the past 1.5-ish years. I like to customize my OS slightly, to give it more personality. For this reinstall I decided to name my OS mimikyu after my favourite pokemon. Anyhow, during the process of tweaking I had the idea of making the alert/notification sound the “cry” of mimikyu. My Google-fu wasn’t strong enough to able to find a solution online, but I eventually figured it out. Maybe this is of help to someone in the future.

1. Add your sound as an .ogg file to the /usr/share/sounds/gnome/default/alerts/. For me /usr/share/sounds/gnome/default/alerts/mimikyu-cry.ogg
2. Next edit the file /usr/share/gnome-control-center/sounds/gnome-sounds-default.xml to add your new sound. This is how the sound prefences UI will detect your new alert. The file should be a pretty straight forward XML file. Add your custom sound something like so:

<sound deleted="false">
    <name>Mimikyu Cry</name>
    <filename>/usr/share/sounds/gnome/default/alerts/mimikyu-cry.ogg</filename>
</sound>

3. Next just select your new alert in the preferences dialog (Gnome Settings -> Sound -> Sound Effects)