So I have Notes 8.5.x installed on my Mac and the new mail tune that plays by default is too loud and really nasty. So I set about finding how to change it. I found this setting hidden in the hundreds of options we now have to play with in the "Preferences"section of the menus.
So to get to this screen you need to press the Apple key and comma in the Notes client. Then select the Mail drop down arrow and "Sending and Receiving". The screen should look like this one below.
Highlighted you will see the "Browse" button that will let you pick a new tune.
Enjoy the nice Apple sounds.