I lost my Mail sounds again--incoming and outgoing

I have lost my whoosh and ping sounds again. I'm talking about the sounds I'm supposed to hear when mail comes in and when I send an E-mail out. This has happened before, but I have always fixed the problem by going to MAIL>Preferences>General and then selecting a sound for incoming messages and putting a checkmark in the "play sounds for other actions" box. This time I found that the settings were already correct, so I'm puzzled. My speakers are working because I can play music from my iTunes library. What could the problem be? Of course I am curious about how this could have happened, but I'm really more interested in getting the sounds back.