Hi @Pck2, welcome to our community!
In most applications you change the notification sound directly in the settings for the application.
If you want to change the notification sound for the Email application, start with opening up the app > 3 dots in the top right corner > Settings > tap on your account > Notifications. Here you can tap on Notification sound and choose a different one.
It works very similar in the Messaging application, open up the app > 3 dots in the top right corner > Settings > Notification sound.
Let me know how it goes! 
