Well, maybe I have an answer slightly smarter than the previous one...
(however, I am not an expert about this audio/video codec stuff, so I may well be wrong)
From Plextools' Help:
Multimedia Player uses the MCI drivers installed on your system to play Audio tracks and other types of Multimedia files. The list of possibilities with Multimedia Player varies from system to system, depending upon the installed MCI devices
Check if your system has a codec for mp3 files: