Open iTunes and create a playlist with the M4P files you want to convert to MP3.
Insert a blank CD into you computer.
Click on the "Burn Disc" button in the bottom right corner of iTunes. Select "Audio CD" as the disc format and click "burn." iTunes will burn a traditional audio CD with the M4P tracks you selected.
Import your newly created CD into iTunes, you can choose to overwrite your existing M4P files or leave them be. The files imported from the CD will not be encoded with DRM, and can be treated like any other MP3 file.