Notification sounds for AOL mail not working on my imac

Yes, my computer is plugged in and powered on.

Yes, I have checked "Play a sound when new mail arrives" in my AOL Mail Settings.

No, my computer is not muted.

I hear all other sounds from all other sites, it is only the AOL sounds that do not work.

Also, next to "Play a sound when new mail arrives" there is a Preview option. Nothing happens when I click on it.

Does anyone know how to fix this?

Thanks.