Ubuntu 12.04 install ffmpeg with amr support

1)Install the source package
apt-get source ffmpeg


2) install dependencies to build the package
apt-get build-dep ffmpeg


3) Install dependencies for custom options (confflags)
apt-get install libopencore-amrnb-dev libopencore-amrwb-dev

4) ffmpeg is on of the components of libav so apt-get source ffmpeg will create /usr/src/libav-0.8.3 with the source so cd to that location
cd /usr/src/libav-0.8.3

4) change confflags to enable some extra options for ffmpeg
vim debian/confflags
add following lines in '# Common configuration flags' section
confflags += --enable-libopencore-amrwb
confflags += --enable-libopencore-amrnb
confflags += --enable-version3

5) Build the package
dpkg-buildpackage -rfakeroot -uc -b

6) Install the resulting deb package(s)

cd /usr/src

dpkg -i ffmpeg_0.8.3-0ubuntu0.12.04.1_all.deb

7) Using ffmpeg to convert mp3 to amr
ffmpeg -y -vn -i in_file.mp3 -y -acodec libvo_amrwbenc -ac 1 -ar 16000 -ab 7.4k -f amr -map_metadata  -o out_file.amr