From the announcement of MPlayer 1.0pre8 [June 11, 2006]:

"Among the many features that have been added, it should be noted that
libavformat now supports a greater  variety  of  files.  If you have a
hard time playing some files, please  try  demuxing  with  libavformat
(-demuxer lavf). MEncoder can also  take advantage of libavformat,  in
order to, for example, create Flash videos."