You can get the "All" codecs package
here.
From the included README:
Code:
Put the files contained in this archive in a directory where MPlayer will find
them. The default directory is /usr/local/lib/codecs/ ($prefix/lib/codecs/) if
you are compiling from source, but you can change that value by passing the
'--with-codecsdir' option to './configure'.
If you use a prebuilt MPlayer package it will most likely be /usr/lib/codecs,
see the documentation of your package for details.
In the past /usr/local/lib/win32 or /usr/lib/win32 was the default directory,
some packages as well as a few other Unix players like xine and avifile still
use it, refer to their documentation for further details.