Just been messing about and managed to create a batch file script using ffmpeg. The script also takes into account spaces in file names.

So now you can take all those .mp4(.flv) that youve got saved in a single folder, strip out the video and be left with the sound file.

#!/bin/sh
for m in *.mp4 ;
do FILE=${m%%.mp4};
ffmpeg -i "$FILE.mp4" -acodec copy -vn "$FILE.m4a"
done