Unless I am severely mistaken, the D80 is a dSLR and hence, its "shutter sound" is exactly that, the sound of the shutter's mechanical action. The mirror, (or prism) flips out of position, the shutter leaves snap open, you capture the image. On film cameras like my N80, this is also accompanied by the sound of the film drive. These are all necessary parts of the camera's operation, and each action creates a sound. No sound, no picture.
Some digicams have digital shutters, (the sensors are always open) and therefore lack the characteristic shutter sound. I have an old Kodak DX6490 that has the option of creating a shutter sound each time you press the button, but it really has nothing to do with the way the camera works.
This is just a simplification of the issue, I'm sure that others can state it more accurately. In short, a dSLR will
create a sound, a digicam may
I hope this is of help.
Tom, on Point Pelee, Canada
-edit- while I was composing my reply, d_oshea beat me to the punch. His answer is right on the mark, and much more concise.