Source: http://lifehacker.com/5530755/free-audio-editor-strips-audio-from-youtube-and-video-files
By entering a URL, you can strip audio from YouTube videos online. It downloads the YouTube video in FLV, then strips the audio from the FLV file. Free Audio Encoder will also strip audio from normal videos, such as those in AVI format.
We've covered Free Audio Editor before, and discovered the program was powerful and intuitive with its streamlined interface. The newest version remains a good pick for getting basic audio work done, and still offers rich features such as CD ripping/burning, text-to-speech conversion, and audio editing. While open-source audio editing programAudacity is in the same league, Free Audio Editor's updates give it a different use case for those grabbing what they can from the web.
Free Audio Editor is freeware for Windows only. A Deluxe version goes for $29.99 and unlocks a few obscured features, but the basic version will get you very far.