Using Outlook for Android in business environments for email and scheduling
Same thing here (Xaiomi Mi Note 3 10.0.3.2)
I had been receiving emails and getting sounds and with no intervention on my part they stopped. I saw my phone light up but there was no sound...
...I put it down to a glitch and left it. All the next day it had no sound on receiving emails, I eventually checked the setting in Outlook (Android platform, official app) and it said :-
New email sound
None
Which I couldn't change as it takes me to the wrong screen, the Sent Email sound works correctly.
I looked back in the updates list from Google Play and found an Outlook update round about the time that this issue started. Which would be update 3.0.107 (331)
I uninstalled and reinstalled and the sound is back to default but you cannot change it as it still takes me to the wrong screen as shown in this video.