ffmpeg -i Florence\ and\ The\ Machine\ -\ Shake\ It\ Out.mp4 -acodec libfaac -ab 128k Florence\ and\ The\ Machine\ -\ Shake\ It\ Out.aac
ffmpeg version 0.9, Copyright (c) 2000-2011 the FFmpeg developers
  built on Dec 27 2011 02:17:27 with llvm_gcc 4.2.1 (Based on Apple Inc. build 5658) (LLVM build 2336.1.00)
  configuration: --prefix=/opt/local/ --enable-libmp3lame --enable-libfaac --enable-nonfree --incdir=/opt/local/include --extra-ldflags=-L/opt/local/lib --extra-cflags=-I/opt/local/include
  libavutil    51. 32. 0 / 51. 32. 0
  libavcodec   53. 42. 0 / 53. 42. 0
  libavformat  53. 24. 0 / 53. 24. 0
  libavdevice  53.  4. 0 / 53.  4. 0
  libavfilter   2. 53. 0 /  2. 53. 0
  libswscale    2.  1. 0 /  2.  1. 0
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'Florence and The Machine - Shake It Out.mp4':
  Metadata:
    major_brand     : mp42
    minor_version   : 0
    compatible_brands: isommp42
    creation_time   : 2011-10-02 17:34:48
  Duration: 00:04:41.88, start: 0.000000, bitrate: 4307 kb/s
    Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 1920x1080, 4154 kb/s, 25 fps, 25 tbr, 1k tbn, 50 tbc
    Metadata:
      creation_time   : 1970-01-01 00:00:00
      handler_name    : VideoHandler
    Stream #0:1(und): Audio: aac (mp4a / 0x6134706D), 44100 Hz, stereo, s16, 148 kb/s
    Metadata:
      creation_time   : 2011-10-02 17:34:50
      handler_name    :
File 'Florence and The Machine - Shake It Out.aac' already exists. Overwrite ? [y/N] y
Output #0, adts, to 'Florence and The Machine - Shake It Out.aac':
  Metadata:
    major_brand     : mp42
    minor_version   : 0
    compatible_brands: isommp42
    creation_time   : 2011-10-02 17:34:48
    encoder         : Lavf53.24.0
    Stream #0:0(und): Audio: aac, 44100 Hz, stereo, s16, 128 kb/s
    Metadata:
      creation_time   : 2011-10-02 17:34:50
      handler_name    :
Stream mapping:
  Stream #0:1 -> #0:0 (aac -> libfaac)
Press [q] to stop, [?] for help
size=    4298kB time=00:04:41.84 bitrate= 124.9kbits/s