After upgrading I found an anomaly with the "Sounds" associated.

In 8.1 the sounds were "hidden" under the "MailBeep" keys in Regedit.  The sounds applet did not accept custom sounds in the "Desktop Mail Notification" event.

In some builds of WPTP the Sounds translated.  When I upgraded to WIN 10 (10240) the registry correctly tracked my custom sounds.

With this build the sounds reverted to "Windows Notify Email." So I manually set the values under the "MailBeep" key.

I used this "script" (file.reg for example) to change back to my preference.  Substitute your custom sound for "Mail2.wav"

Be sure to create a restore point before working with Regedit!

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Windows Registry Editor Version 5.00

[HKEY_CURRENT_USERAppEventsSchemesApps.DefaultMailBeep.Current]
@="C:\Windows\Media\mail2.wav"

[HKEY_CURRENT_USERAppEventsSchemesApps.DefaultMailBeep.Default]
@="C:\Windows\Media\mail2.wav"

[HKEY_CURRENT_USERAppEventsSchemesApps.DefaultMailBeepCustom]
@="C:\Windows\media\Mail2.wav"

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There may be other older sound schemes.

HTH.