ffmpeg -i thirty.flv -acodec libmp3lame -ab 26k output/thirty.mp3
ffmpeg version N-37063-g14d94a1 Copyright (c) 2000-2012 the FFmpeg developers
  built on Jan 23 2012 13:26:15 with llvm_gcc 4.2.1 (Based on Apple Inc. build 5658) (LLVM build 2335.15.00)
  configuration: --enable-libmp3lame
  libavutil      51. 34.101 / 51. 34.101
  libavcodec     53. 57.105 / 53. 57.105
  libavformat    53. 30.100 / 53. 30.100
  libavdevice    53.  4.100 / 53.  4.100
  libavfilter     2. 59.101 /  2. 59.101
  libswscale      2.  1.100 /  2.  1.100
  libswresample   0.  6.100 /  0.  6.100
[flv @ 0x7fc60a83d800] Could not find codec parameters (Video: none)
Input #0, flv, from 'thirty.flv':
  Metadata:
    canSeekToEnd    : true
  Duration: 00:34:35.04, start: 3.040000, bitrate: 20 kb/s
    Stream #0:0: Video: none, 1k tbr, 1k tbn, 1k tbc
    Stream #0:1: Audio: nellymoser, 44100 Hz, mono, s16
File 'output/thirty.mp3' already exists. Overwrite ? [y/N] y
Output #0, mp3, to 'output/thirty.mp3':
  Metadata:
    canSeekToEnd    : true
    TSSE            : Lavf53.30.100
    Stream #0:0: Audio: mp3, 44100 Hz, mono, s16, 26 kb/s
Stream mapping:
  Stream #0:1 -> #0:0 (nellymoser -> libmp3lame)
Press [q] to stop, [?] for help
size=     756kB time=00:03:13.46 bitrate=  32.0kbits/s
video:0kB audio:756kB global headers:0kB muxing overhead 0.028429%