Not possible I'm afraid.. there's no direct relationship between a .swf file and it's orginal source file. In fact, there's not even a way to determine what format the source code was in (flash,livemotion,proprietary).
On the OTHER hand, you can reduce it to it's primary elements (pictures/sounds/actionscripts) -- try this:
http://www.swifftools.com/stools/ -- download SWF browser. You may also need a flash deprotecter depending on if it's password protected.