ffmpeg -i test_voice.amr -acodec mp3 -ac 1 -ar 16000 test_voice.mp3

yum install nasm
cd /usr/local/src
wget http://downloads.sourceforge.net/lame/lame-3.100.tar.gz
tar -zxvf lame-3.100.tar.gz
cd lame-3.100
./configure --prefix=/usr/local
make && make install
ln -s /usr/local/lib/libmp3lame.so.0.0.0 /usr/lib64/libmp3lame.so.0
wget http://ffmpeg.org/releases/ffmpeg-snapshot.tar.bz2
tar -jxvf ffmpeg-snapshot.tar.bz2
cd ffmpeg
./configure --prefix=/usr/local --enable-libmp3lame
make && make install
ffmpeg