As of macOS Sierra, assigning a custom sound file in Mail no longer appears possible. All I get is “New Messages Sound” in addition to the old options:
Is there a way to assign a custom Mail sound in macOS Sierra, possibly via editing a plist?
The interesting thing is, I’ve logged into the Guest account, and even though the same option is selected, the sound that plays when a new mail comes in is the sound I had previously under MY user account. But I can’t seem to find where that sound file is assigned.