The following command can be used to use FFMPEG to extract the audio from video files to separate MP3 files. If you want to extract the audio from a file type other then FLV change the value of /m as appropriate.
forfiles /m *.flv /c "cmd /c ffmpeg -i @FILE @FILE.mp3"
The end result will be for every video file you will end up with an audio file of the same name with a .mp3 extension. i.e. video.flv and video.flv.mp3
If you would rather have video.flv and video.mp3 you should substitute the @FILE.mp3 for @FNAME.mp3
forfiles /m *.flv /c "cmd /c ffmpeg -i @FILE @FNAME.mp3"