FFmpeg can be patched to enable 3gp conversions, useful for putting video onto mobiles.  If the source name as amr in it then this has been done, otherwise use these patches:

wget http://www.3gpp.org/ftp/Specs/archive/26_series/26.073/26073-510.zip
wget http://www.3gpp.org/ftp/Specs/archive/26_series/26.104/26104-510.zip
wget http://www.3gpp.org/ftp/Specs/archive/26_series/26.204/26204-510.zip

26073-510.zip into libavcodec/amr
26104-510.zip into libavcodec/amr_float
26204-510.zip into libavcodec/amrwb_float

Install ffmpeg
>./configure --prefix=/usr/local --enable-shared --enable-pp --enable-swscaler --enable-pthreads --enable-x11grab --enable-a52 --enable-dts --enable-faac --enable-mp3lame --enable-libogg --enable-libtheora --enable-vorbis --enable-x264 --enable-xvid --enable-amr_nb --enable-gpl --cpu=pentium4
>make && make install && ldconfig



Unpack mplayer into /usr/local/src

Unpack ffmpeg and copy the libavcodec, libavutil and libavformat subdirectories into /usr/local/src/mplayer

Install mplayer with:
>CFLAGS= ./configure --prefix=/usr/local --enable-gui --with-extralibdir=/usr/lib/win32
>make
>make install && ldconfig
>ln -s /usr/share/fonts/DejaVuSans.ttf /home/username/.mplayer/subfont.ttf

Unpack the font package to /usr/local/share/mplayer/font

Unpack the skins packages into /usr/local/share/mplayer/skins