x86_64 version needs corresponding 64 bit codecs,that is key to install mplayer in a x86-64 version linux.

So, firstly we need to download those codecs.

download below listed codecs from here:http://www.mplayerhq.hu/MPlayer/releases/codecs/

all-20071007.tar.bz2
essential-amd64-20071007.tar.bz2
rp9codecs-win32-20050115.tar.bz2
essential-20071007.tar.bz2
rp8codecs-win32-20040626.tar.bz2
windows-all-20071007.zip

Secondly,extract above codecs you downloaded:

tarj -jxvf all-20071007.tar.bz2
tarj -jxvf essential-amd64-20071007.tar.bz2
tarj -jxvf rp9codecs-win32-20050115.tar.bz2
tarj -jxvf essential-20071007.tar.bz2
tarj -jxvf rp8codecs-win32-20040626.tar.bz2
unzip -u windows-all-20071007.zip

and then,copy all files to /opt/mplayer/codecs

cp all-20071007/* /opt/mplayer/mycodecs/
cp essential-amd64-20071007/* /opt/mplayer/mycodecs/
cp rp9codecs-win32-20050115/* /opt/mplayer/mycodecs/
cp essential-20071007/* /opt/mplayer/mycodecs/
cp rp8codecs-win32-20040626/* /opt/mplayer/mycodecs/
cp windows-all-20071007/* /opt/mplayer/mycodecs/

If you have installed Realplayer,you still need to copy all files of essential-amd64-20071007 in to /usr/lib64/RealPlayer10/codecs/