You can do with any software that you want!! there is not any great difference between them!! I use "Jet Audio" for converting files!! It's good!! if you set those values for output file in every softwares, you will get a same mp3 file!! "There is no difference in converting by each software!!"
then, the original flash video file
is not a joint stereo!!
May I ask, Why is it so important for you to keep that mp3 untouched?
