avconv -i input.flv -vn -ab 192k output.mp3
avconv version 0.8.8-4:0.8.8-0ubuntu0.12.04.1, Copyright (c) 2000-2013 the Libav developers
  built on Oct 22 2013 12:31:55 with gcc 4.6.3
[flv @ 0x20319a0] Estimating duration from bitrate, this may be inaccurate
Input #0, flv, from 'input.flv':
  Metadata:
    starttime       : 0
    totalduration   : 206
    totaldatarate   : 252
    bytelength      : 6514627
    canseekontime   : true
    sourcedata      : B4A7D0DE2HM1383758923570386
    purl            : 
    pmsg            : 
  Duration: 00:03:26.16, start: 0.000000, bitrate: 250 kb/s
    Stream #0.0: Video: h264 (Main), yuv420p, 640x480 [PAR 1:1 DAR 4:3], 131 kb/s, 25 tbr, 1k tbn, 50 tbc
    Stream #0.1: Audio: aac, 44100 Hz, stereo, s16, 118 kb/s
Output #0, mp3, to 'output.mp3':
  Metadata:
    starttime       : 0
    totalduration   : 206
    totaldatarate   : 252
    bytelength      : 6514627
    canseekontime   : true
    sourcedata      : B4A7D0DE2HM1383758923570386
    purl            : 
    pmsg            : 
    TSSE            : Lavf53.21.1
    Stream #0.0: Audio: libmp3lame, 44100 Hz, stereo, s16, 192 kb/s
Stream mapping:
  Stream #0:1 -> #0:0 (aac -> libmp3lame)
Press ctrl-c to stop encoding
size=    4846kB time=206.73 bitrate= 192.0kbits/s    
video:0kB audio:4845kB global headers:0kB muxing overhead 0.007296%