First of all, youtube shows video clips and not audio clips. When you grab a file from YouTube it’s usually an .FLV file. You may grab it from online services like GrabFlv.com or use applications similar to TubeTV. The FLV is a video file and we need to grab the audio portion of it. The software I use is call iExtractMP3. This software feature a drag and drop and single button clicking to extract (NOTE: I emphasize the term because the software uses extraction method rather than re-encode the audio. This results in higher quality audio since whatever the poster put, this is the same file you get).
Alternatively, there’s also another file conversion service offered by Zamzar. I have not used the service before, but it seems that the process is simply point the URL to the video that you want and you’ll receive the link for the audio.