e.g., akx has created some nice ones:
wget -O Unphased.ogg https://github.com/akx/Notifications/blob/master/OGG/Unphased.ogg
wget -O Modular.ogg https://github.com/akx/Notifications/blob/master/OGG/Modular.ogg
Those two are already configured in example settings, modify audio_files in your settings.json accordingly if you want to use different ones.