title: Can't extract mp3 audio from video with ffmpeg
body: <p>My input is a .mp4 video file.</p>
  
  <p>Try #1</p>
  
  <pre><code>ffmpeg -i pipe:0 -y -map a media/73.mp3
  </code></pre>
  
  <p>Error:</p>
  
  <pre><code>ffmpeg version 3.3.3 Copyright (c) 2000-2017 the FFmpeg developers
    built with Apple LLVM version 8.1.0 (clang-802.0.42)
    configuration: --prefix=/usr/local/Cellar/ffmpeg/3.3.3 --enable-shared --enable-pthreads --enable-gpl --enable-version3 --enable-hardcoded-tables --enable-avresample --cc=clang --host-cflags= --host-ldflags= --enable-libmp3lame --enable-libx264 --enable-libxvid --enable-opencl --enable-videotoolbox --disable-lzma --enable-vda
    libavutil      55. 58.100 / 55. 58.100
    libavcodec     57. 89.100 / 57. 89.100
    libavformat    57. 71.100 / 57. 71.100
    libavdevice    57.  6.100 / 57.  6.100
    libavfilter     6. 82.100 /  6. 82.100
    libavresample   3.  5.  0 /  3.  5.  0
    libswscale      4.  6.100 /  4.  6.100
    libswresample   2.  7.100 /  2.  7.100
    libpostproc    54.  5.100 / 54.  5.100
  [mov,mp4,m4a,3gp,3g2,mj2 @ 0x7f977d800000] stream 1, offset 0xa8: partial file
  [mov,mp4,m4a,3gp,3g2,mj2 @ 0x7f977d800000] Could not find codec parameters for stream 0 (Video: h264 (avc1 / 0x31637661), none(smpte170m/smpte170m/bt709), 320x176, 300 kb/s): unspecified pixel format
  Consider increasing the value for the 'analyzeduration' and 'probesize' options
  Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'pipe:0':
    Metadata:
  major_brand     : mp42
  minor_version   : 0
  compatible_brands: mp42isomavc1
  creation_time   : 2012-03-13T08:58:06.000000Z
  encoder         : HandBrake 0.9.6 2012022800
    Duration: 00:00:10.03, bitrate: N/A
  Stream #0:0(und): Video: h264 (avc1 / 0x31637661), none(smpte170m/smpte170m/bt709), 320x176, 300 kb/s, 25 fps, 25 tbr, 90k tbn, 180k tbc (default)
  Metadata:
  creation_time   : 2012-03-13T08:58:06.000000Z
    encoder         : JVT/AVC Coding
  Stream #0:1(und): Audio: aac (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 160 kb/s (default)
  Metadata:
    creation_time   : 2012-03-13T08:58:06.000000Z
  Stream #0:2(und): Audio: aac (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 160 kb/s
  Metadata:
    creation_time   : 2012-03-13T08:58:06.000000Z
  Stream #0:3(und): Subtitle: mov_text (text / 0x74786574), 0 kb/s
  Metadata:
    creation_time   : 2012-03-13T08:58:06.000000Z
  Stream mapping:
    Stream #0:1 -&gt; #0:0 (aac (native) -&gt; mp3 (libmp3lame))
    Stream #0:2 -&gt; #0:1 (aac (native) -&gt; mp3 (libmp3lame))
  [mov,mp4,m4a,3gp,3g2,mj2 @ 0x7f977d800000] stream 1, offset 0xa8: partial file
  pipe:0: Invalid data found when processing input
  Finishing stream 0:0 without any data written to it.
  Finishing stream 0:1 without any data written to it.
  [mp3 @ 0x7f977b846800] Invalid audio stream. Exactly one MP3 audio stream is required.
  Could not write header for output file #0 (incorrect codec parameters ?): Invalid argument
  Error initializing output stream 0:1 -- 
  Conversion failed!
  </code></pre>
  
  <hr>
  
  <p>Try #2</p>
  
  <pre><code>ffmpeg -i pipe:0 -y -vn -acodec copy media/72.mp3
  </code></pre>
  
  <p>Error:</p>
  
  <pre><code>[...]
  [mp3 @ 0x7febed835200] Invalid audio stream. Exactly one MP3 audio stream is required.
  Could not write header for output file #0 (incorrect codec parameters ?): Invalid argument
  </code></pre>
  
  <hr>
  
  <p>Try #3</p>
  
  <pre><code>ffmpeg -i pipe:0 -y -codec:a libmp3lame -qscale:a 2 media/74.mp3
  </code></pre>
  
  <p>No error with <code>ffmpeg</code>, but the file is invalid (it's size is 334 bytes)</p>
  
  <hr>
  
  <p>Try #4</p>
  
  <pre><code>ffmpeg -i pipe:0 -y -vn -acodec mp3 media/78.mp3
  </code></pre>
  
  <p>No error with <code>ffmpeg</code>, but the file is invalid (it's size is 334 bytes)</p>
  
  <hr>
  
  <p>Try #5</p>
  
  <pre><code>ffmpeg -f mp4 -i pipe:0 -y media/80.mp3
  </code></pre>
  
  <p>No error with <code>ffmpeg</code>, but the file is invalid (it's size is 334 bytes)</p>
  
  <hr>
  
  <p>What am I doing wrong? This seems like a fairly easy thing to do...</p>