If you hook your line out to your mic then you should get pretty decent stereo from it, then yes you can save it as a .wav with Windows Sound Recorder and if you have a .mp3 converter you can get your sought after .mp3.
However, that does seem to be one hell of a long way round. Can't you just open the .swf in flash and extract the audio track or something? I don't know how flash works because I never tried to learn it (inaccessible).