Utube Ripper is an application written in Gambas that works exclusively on Linux. It's useful to download and convert Youtube videos in a simple and efficient way. You can also convert them into a non-flash format. You can use the "Rip audio only" option to extract audio from a YouTube video and can convert into an MP3 format.
Download .deb file from here
now double-click on deb file and install with gdebi installer.
or open a terminal and use sudo dpkg -i filename.deb