Sothink SWF Decompiler é o melhor disponível para isso, mas custa um pouco:
SWF Decompiler Features
Decompile SWF to FLA.
Decompile SWF to FLEX project, and get MXML file and other project files.
Decompile the standard EXE made by Adobe Flash to FLA format.
Support Flash 6, Flash MX 2004 (V7), Flash 8, Flash CS3 (V9) and Flash CS4 (V10).
Compatible with ActionScript 2 and ActionScript 3.
Export SWF to FLA/SWF to FLEX projects, and extract resources in batch mode.
Support exporting the resources to SWF or FLA format.
Extract element into various formats, including shapes, sounds, images, sprites, fonts, texts, ActionScript, etc.
Export ActionScript file in AS, BIN or HTML format.
Export the video file in FLV format from Flash movie.
Multi-Language interface is available, English, German and traditional Chinese included.
Uma alternativa gratuita é o Decompile Flash Free Version