Personal (POP / IMAP) Email Notifications - Nokia Lumia 928
From the Start screen, swipe left to display the Apps list. Alternatively, tap the Right arrow (located in the lower-right; may require scrolling to the bottom of the Start screen).
Tap Settings.
Tap ringtones+sounds.
Tap manage app sounds.
Tap the appropriate email account.
Tap the 'Notfication sound' Dropdown menu.
Choose an alert. To preview, tap the Play icon (located to the left of the corresponding alert).