That depends on what you intend to do with them.  Anything you're likely to download from iTunes will be copyrighted material.  You can use it in MMM for your own personal use, but that is it.  You, technically, cannot share it, post it the web or play in any other public forum, or sell.

It's my understanding that iTunes uses a propietary file format.  You would need to convert the files to a format MMM supports.