On itunes if you click on the individual song you want to change to mp3, then it will go to another screen where it is just that song. Then if you right click the song, and choose "Create MP3 version" this will make another copy of the song but in mp3 format. This works on my itunes not sure if other versions are different or not.
As for changing youtube links to mp3 version there is a good site for this, media converter. Just copy your youtube address from the video into this site and follow the steps.