Byyoutube-dl is a small command-line program to download videos from YouTube.com and a few more sites. To use youtube-dl you only needed to installed Python interpreter, version 2.x (x being at least 4), and it is not platform specific.
youtube-dl will work in your Unix box, in Windows or in Mac OS X. How about the licence? youtube-dl is is released to the public domain, which means you can modify it, redistribute it or use it however you like.
If you have successfully download as flv format and needed to convert as MP3 you can try this command (it’s worked on Linux, we didn’t know work on other Operating System)
[php]for f in *.flv; do ffmpeg -i "$f" -acodec libmp3lame -ab 128k "${f%.flv}.mp3"; [/php]
This command is created by Sufehmi here