emms with mplayer plays most flac files just fine but no sound on some other flac files. without emms, mplayer plays all of them.
with the suggestions of "yrk" (on #emacs) i was able to play those too.
it seems emms falls back to other players when the flac is 24bit (i had emms-player-list set to '(emms-player-mplayer emms-player-ogg123 emms-player-mpg321)", so i removed other players from the list and now it plays those files as well.
thank you.