Hello Scott,
You can choose the ringtone that you want by giving the Outlook app permission to see the files on your notifications folder. To customize the notification sound, copy the sound file to the Notification folder on your file navigator. After that, give permission to the Outlook app to read from the storage by checking your Settings > Apps > Outlook > App permissions/Permission > Storage > Enable. You should be able to choose the sound file as your notification sound on the Outlook app.
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