- You need a audio convert software named "TuneClone" first, you need download and install TuneClone Audio Converter free
trial version.then just follow the screen tips to install TuneClone Audio Converter, this software will install a virtual CD Drive on your PC, and convert protected
music files by using this virtual CD Burner.

- After installation, please open iTunes, and select menu "Edit" and select menu item "Preferences..."
- Set iTunes CD burner to "TuneClon Virtual CD_RW".
- Create a playlist for the M4P files that you want to convert to MP3 files.

- Click "Burn Disc" button to convert M4P to MP3 with virtual CD burning.
- After the playlist has been burned. you will get the converted MP3 files in TuneClone's main window.
- Users can make a maximum of seven CD copies of any particular playlist containing songs in the m4p file format purchased from the iTunes Store.
- Users can access their purchased songs in the m4p file format on a maximum of five computers.
- Songs in the m4p file format can only be played on a computer with iTunes or an iPod; other MP3 devices do not support FairPlay encoded tracks.
Since TuneClone uses a virtual cd drive to convert music files, it also supports almost all the music formats that your media player software supports. No matter it is protected or not.
More about iTunes protected music files:
Apple's FairPlay digital rights management (DRM) is integrated into iTunes, which manages songs purchased in the m4p file format from iTunes Store before January 2009. iTunes relies on FairPlay to implement three main restrictions:
There are no restrictions on number of iPods to which a purchased song can be transferred nor the number of times any individual song can be burned to CD. When Apple initially introduced FairPlay, songs purchased through iTunes had limits of three simultaneous machines and ten CD copies of a playlist. The adjustment to the current limits was implemented with the introduction of iTunes 4.5 in April 2004, presumably as the result of re-negotiations Apple had with major labels. To remove Apple's DRM, a user can convert protected files to an unprotected format format by using iTunes to burn them to an audio CD and ripping the CD back to iTunes. however, this method could not preserve the music's meta information, such as album name, artist,gerne etc. TuneClone simulate the cd-burning and ripping process by using a virtual cd and can keep all the information of the songs, even the album photo.
More info about Apple's DRM, please read here