Generally speaking, multimedia playlists cannot be converted to multimedia files. A .mpl file contains just a list of videos or music to be played in a sequence with a media player, not the actual multimedia files. As such, mpl to mp4 conversion is not possible.
However, you can manually open the playlist using a text editor and check the location of the actual audio files. If the playlist is not yours, it is unlikely you will find them thought.Open MPL file Open MP4 file
Suite of tools for video editing
Roxio Creator is set of tools used to burn CDs, DVDs and Blu-ray discs
Roxio Toast 11 Titanium makes disc burning, DVD copying and video conversion as easy as drag, drop and go