It is unlikely for mplayer to support real files on Nokia 770. On PC mplayer is able to use closed source binary codec which is compiled for x86, it just will not work on arm.

I guess, information in .deb file description is somewhat misleading, does not quite apply to maemo port and should be edited (at least parts about win32 dll's and 3dfx).

Also unfortunately MPlayer on Nokia 770 has problems with AAC and AC3 audio (they are either buggy or unusable because of floating point math being too slow). We need to either find and use integer only decoders for them or use DSP based gstreamer decoder for AAC.