If you already have ffmpeg installed (without mp3 support), then firstly uninstall it.
MP3 support isn't included with ffmpeg in the default Ubuntu repositories so to get it you have to enable the Medibuntu repos (assuming this is legal in your area).
sudo wget http://www.medibuntu.org/sources.list.d/hardy.list -O /etc/apt/sources.list.d/medibuntu.list
wget -q http://packages.medibuntu.org/medibuntu-key.gpg -O - | sudo apt-key add - && sudo apt-get update
Then you can install ffmpeg and the libmp3lame codec (part of the libavcodec package)
sudo apt-get install ffmpeg
sudo apt-get install libavcodec1d
You can verify the dependencies of ffmpeg with ldd
paul@paul:~/Videos$ ldd /usr/bin/ffmpeg
linux-gate.so.1 => (0xb7f07000)
libavformat.so.1d => /usr/lib/libavformat.so.1d (0xb7e72000)
libavcodec.so.1d => /usr/lib/libavcodec.so.1d (0xb7997000)
libavutil.so.1d => /usr/lib/libavutil.so.1d (0xb798c000)
libm.so.6 => /lib/tls/i686/cmov/libm.so.6 (0xb7967000)
libpthread.so.0 => /lib/tls/i686/cmov/libpthread.so.0 (0xb794f000)
libc.so.6 => /lib/tls/i686/cmov/libc.so.6 (0xb7800000)
libz.so.1 => /usr/lib/libz.so.1 (0xb77eb000)
libogg.so.0 => /usr/lib/libogg.so.0 (0xb77e6000)
libdc1394_control.so.13 => /usr/lib/libdc1394_control.so.13 (0xb77d6000)
libdl.so.2 => /lib/tls/i686/cmov/libdl.so.2 (0xb77d2000)
liba52-0.7.4.so => /usr/lib/liba52-0.7.4.so (0xb77c7000)
libgsm.so.1 => /usr/lib/libgsm.so.1 (0xb77ba000)
libmp3lame.so.0 => /usr/lib/libmp3lame.so.0 (0xb7725000)
libtheora.so.0 => /usr/lib/libtheora.so.0 (0xb76db000)
libvorbis.so.0 => /usr/lib/libvorbis.so.0 (0xb76b3000)
libvorbisenc.so.2 => /usr/lib/libvorbisenc.so.2 (0xb75ba000)
libxvidcore.so.4 => /usr/lib/libxvidcore.so.4 (0xb74a2000)
libx264.so.57 => /usr/lib/libx264.so.57 (0xb7417000)
libfaac.so.0 => /usr/lib/libfaac.so.0 (0xb7406000)
/lib/ld-linux.so.2 (0xb7f08000)
libraw1394.so.8 => /usr/lib/libraw1394.so.8 (0xb73ff000)
libX11.so.6 => /usr/lib/libX11.so.6 (0xb7318000)
libxcb-xlib.so.0 => /usr/lib/libxcb-xlib.so.0 (0xb7316000)
libxcb.so.1 => /usr/lib/libxcb.so.1 (0xb72fe000)
libXau.so.6 => /usr/lib/libXau.so.6 (0xb72fa000)
libXdmcp.so.6 => /usr/lib/libXdmcp.so.6 (0xb72f5000)
It should then let you encode videos with mp3 audio...
ffmpeg -i jonathan-davis-news24-2008-01-22.avi -f flv -s 400s220 -acodec mp3 -ar 44100 jd.flv