Hi EZ-D,
We replicate the issue you are having with Outlook here on our end. We tried to select New mail sound and Send mail sound in Outlook for Android app and we did not encounter any issues.

To provide an accurate solution, we need to know which version of Outlook is installed on your phone.
For initial troubleshooting, we suggest that you uninstall and then reinstall your Outlook app.
To notify us and have us get back to you, simply reply to this post.
Hi Darleen,
I have the same problem, unable to set the 'New Email sound'. I followed the suggestion of uninstalling and re-installing. Got version 3.0.34 (311) and now the 'New Email sound' has defaulted to 'Outlook Email' for both my Outlook account and my IMAP account,
but I cannot change either to a different sound. When I try to change 'New Email sound' on either account I am taken to the Notification Management screen.
I am running Android version 8.0.0 on Huawei P Smart phone.