Eltima Software has released Airy, a Mac OS X app for downloading videos from YouTube and extracting MP3 sounds from them. It requires Mac OS X 10.7 or higher and costs US$19.95.
Airy supports MP4, WebM, FLV, 3GP in various resolutions. If you need to extract the soundtrack from the initial video, enter the link in the Eltima Software app, choose MP3 format and click Download.
A free trial version of Airy allows five free downloads. For more info go to http://tinyurl.com/m7zl4jl .