Step 1 :
Copy the youtube link that you want to download as a audio file.
Example : http://www.youtube.com/watch?v=n_vGArukzq8
Step 2:
Now go to the following site : http://vidtomp3.com/
Step 3 :
You will find a video url box to input your youtube link.Paste the youtube link in that box and create download.
Step 4 :
Wait for a while.It will create the mp3 version of youu youtube file.Now download your youtube music as a mp3 by using a download manager.
NOTE: The Problem is that the downloaded file is big in Size and may not sounds good.so you can convert it as acc format in itunes and load it to your ipod,gadgets or pc's