Using Outlook for Android in business environments for email and scheduling
My understanding on HTC 10 is that for "outgoing mail" notification sound, Outlook probably handles itself the choice of the sound, while for "incoming sound", it let the default phone notification handler provide a choice, and that one is not aware of the default Outlook-provided sounds. To set the default sound notification for a specific app, NotifiCon app might help (although if you have multiple e-mail accounts in Outlook, they will probably all get the same sound set in NotifiCon while Outlook seems to provide a way to set a specific sound for each account, but I am not able to customize my Exchange and Outlook.com accounts, while Office 365 one looks OK...).
So maybe the use of NotifiCon with the sounds retrieved by Francesco could more or less solve the problem described by the OP without resetting the accounts if other settings/methods did not work...