Hmm, unless you made the .swf file, I am pretty sure any way of doing it is illegal. When you make a .swf your molding everything together (much like a code compiler) and there is no legal way to extract bits and pieces.
Is there a song or something that is in a flash file and you want to know what it is? If so you may post a link and maybe someone can help your from there.
There are also tons of sound clips that are free for use over at http://www.flashkit.com
. Maybe you can find something similar?