ffmpeg -i video.mp4 -codec:a copy -vn audio2.mp3
ffmpeg version 2.1.1 Copyright (c) 2000-2013 the FFmpeg developers
  built on Dec 10 2013 06:36:11 with gcc 4.8.2 (GCC)
  configuration: --prefix=/usr --disable-debug --disable-static --enable-avresample --enable-dxva2 --enable-fontconfig --enable-gnutls --enable-gpl --enable-libass --enable-libbluray --enable-libfreetype --enable-libgsm --enable-libmodplug --enable-libmp3lame --enable-libopencore_amrnb --enable-libopencore_amrwb --enable-libopenjpeg --enable-libopus --enable-libpulse --enable-librtmp --enable-libschroedinger --enable-libspeex --enable-libtheora --enable-libv4l2 --enable-libvorbis --enable-libvpx --enable-libx264 --enable-libxvid --enable-pic --enable-postproc --enable-runtime-cpudetect --enable-shared --enable-swresample --enable-vdpau --enable-version3 --enable-x11grab
  libavutil      52. 48.101 / 52. 48.101
  libavcodec     55. 39.101 / 55. 39.101
  libavformat    55. 19.104 / 55. 19.104
  libavdevice    55.  5.100 / 55.  5.100
  libavfilter     3. 90.100 /  3. 90.100
  libavresample   1.  1.  0 /  1.  1.  0
  libswscale      2.  5.101 /  2.  5.101
  libswresample   0. 17.104 /  0. 17.104
  libpostproc    52.  3.100 / 52.  3.100
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'video.mp4':
  Metadata:
    major_brand     : mp42
    minor_version   : 0
    compatible_brands: isommp42
    creation_time   : 2012-02-15 06:03:04
  Duration: 00:20:14.15, start: 0.000000, bitrate: 200 kb/s
    Stream #0:0(und): Video: h264 (Constrained Baseline) (avc1 / 0x31637661), yuv420p, 640x360, 126 kb/s, 15 fps, 15 tbr, 30 tbn, 30 tbc (default)
    Metadata:
      creation_time   : 1970-01-01 00:00:00
      handler_name    : VideoHandler
    Stream #0:1(und): Audio: aac (mp4a / 0x6134706D), 44100 Hz, mono, fltp, 71 kb/s (default)
    Metadata:
      creation_time   : 2012-02-15 06:03:05
      handler_name    : IsoMedia File Produced by Google, 5-11-2011
[mp3 @ 0x2508be0] Invalid audio stream. Exactly one MP3 audio stream is required.
Output #0, mp3, to 'audio2.mp3':
  Metadata:
    major_brand     : mp42
    minor_version   : 0
    compatible_brands: isommp42
    encoder         : Lavf55.19.104
    Stream #0:0(und): Audio: aac (mp4a / 0x6134706D), 44100 Hz, mono, 71 kb/s (default)
    Metadata:
      creation_time   : 2012-02-15 06:03:05
      handler_name    : IsoMedia File Produced by Google, 5-11-2011
Stream mapping:
  Stream #0:1 -> #0:0 (copy)
Could not write header for output file #0 (incorrect codec parameters ?): Invalid argument