I am not sure I understand what you mean. Word-splitting is performed by the "for x in y" construct, using the IFS variable, so by default if you have a file called "foo bar.mp4" the command line from the article:

  for i in *.mp4; do ffmpeg -i "$i" "${i%.mp4}.mp3"; done

will result in executing:

  ffmpeg -i foo foo.mp3
  ffmpeg -i bar.mp4 bar.mp3

Which is obviously not what was meant. So it's a good habit to learn to loop over files in a directory in a different way.