This page in Apple's documentation says:
A custom or system-provided sound can accompany any of the other three notification types.
But I can't find anywhere how to specify a system-provided sound. How do I do this? I want to use one of Apple's built-in sounds in iOS besides the default "tri-tone" one. I've already tried putting in the names of some of the system sounds, and that didn't work. I know how to put custom sounds in, but I don't want to copy an iOS sound file into my app because of copyright.