Using Outlook for Android in business environments for email and scheduling
This issue has been ignored. I can confirm in January of 2020 the issue remains that if you by chance change the new email sound, you will be unable to change it back to the Outlook Email tone as it will not appear in the list of options.
Only the available sounds from the device will appear. This does not occur, however, with the sent E-mail notification option which behaves as it should.
While a better solution would consist of Microsoft actually fixing this, here is what does work: Uninstall and reinstall the app. Any accounts added upon app setup will have outlook's app provided sounds assigned by default.