Some people had problems by Playing VCD-MPEG1-Files with MS Media Player. Here some impossible solutions:

One person wrote me this:
"i got the same problem with the mplvpx.dll file. No more chance to play a video file through Mplayer 6.4 or 7.1. So i searched through my system and came to the conclusion that this file is part of the intel indeo codec. i reinstalled this codec and everything is fine
from now on.

The Codec is downloadable from

http://www.ligos.com/indeo/

Actual Version is 5.11"

I answered him so:
"Thanks, but after reinstalling of Indeo Video i can't also find the mplvpx.dll... But i found an another reason: I installed first MS Media Player 7.0, than 7.1. It works! But the quality is better with the Real
Player."

An another one answered so:
"I have a new solution to the problem. I found out that except for DivX I could also play MPEG1 streams, so I figured out that it had to be the MPEG2
codec and downloaded a new one from http://www.vcdhelp.com which I installed. Now I do not have any problems playing video streams."

One person wrote also that:
"the mplapx.dll and the mplvpx.dll is part of the
lmpgvd.ax codec file.... i found this after hours of searching and scrolling through hex-files: uuuuuuuaaaaaaaaahhhhhhhh "

It seems, the file never exists, it's a part of the lmpgvd.ax file.