We don't have Mplayer (yet), but we have a commandline app, named "play", which uses the same codecs mplayer uses, which can therefore play back anything mplayer can.
Is it "easy" to recompile old Amiga programs to AROS? When porting to MorphOS or AmigaOS4 you mainly finetune your hooks and possibly 68k varargs functions, but I have no idea what kind of precautions are needed for AROS code?