gistfile1.txt

Installing AMR libraries:
Grab the AMR Codec Libraries (The FFmpeg documentation has a section about installing AMR):
e.g. amrnb-6.1.0.4.tar.bz2 and amrwb-7.0.0.1.tar.bz2
Decompress sources:
bunzip2 amrnb-6.1.0.4.tar.bz2
tar xvf amrnb-6.1.0.4.tar
Change directory:
cd amrnb-6.1.0.4
Configure and install:
./configure
make
checkinstall -install=yes
Repeat same steps 2,3 and 4 for amrwb-7.0.0.1.tar.bz2
Type in ldconfig as sudo user to ensure the new modules are available when running ffmpeg
Howto install x264/ffmpeg:

http://ubuntuforums.org/showthread.php?t=786095

- Note: Except for the line on the ./configure for ffmpeg, should be this (adding support for AMR/3GP and GSM):

./configure --enable-gpl --enable-nonfree --enable-pthreads --enable-libfaac --enable-libfaad --enable-libmp3lame --enable-libtheora --enable-libx264 --enable-libxvid --enable-x11grab --enable-libamr-nb --enable-libamr-wb --enable-libgsm

Command for mp4 (Android) to gsm (works for 3gp or mp4 formats):

ffmpeg -i input.3gp -f gsm input.wav