To download a video list in webm format, extract audio and keep the original video file:
youtube-dl -f 43 --extract-audio -k "https://www.youtube.com/playlist?list=AlphanUmerIcCodeofTheList"
Download a video list in preferably free format, otherwise non-free, extracting audio and keeping original video file:
youtube-dl --prefer-free-formats --extract-audio -k
Download a video list in preferably free format, otherwise non-free, extracting audio and keeping original video file, restricting the files names to alphanumeric characters and dashes:
youtube-dl --prefer-free-formats --extract-audio -k --restrict-filenames http://www.youtube.com/playlist?list=LIstCode -o ~/outputDir/%\(title\)s-%\(id\)s.%\(ext\)s