title: How to use ffmpeg to mix two audio files to a single one?
body: <p>I want to mix two audio files to a single one use ffmpeg. So I download the latest version of ffmpeg, and check the document and get the code:</p>
  
  <pre><code>ffmpeg -i 10001.wav -i 201.wav -filter_complex amix=inputs=2:duration=shortest output.wav
  </code></pre>
  
  <p>but I got this error:</p>
  
  <pre><code>ffmpeg version 0.11.1 Copyright (c) 2000-2012 the FFmpeg developers
    built on Aug  9 2012 16:22:59 with gcc 4.1.2 20080704 (Red Hat 4.1.2-52)
    configuration: --prefix=/usr/ --enable-gpl --enable-libx264 --enable-libmp3lame
    libavutil      51. 54.100 / 51. 54.100
    libavcodec     54. 23.100 / 54. 23.100
    libavformat    54.  6.100 / 54.  6.100
    libavdevice    54.  0.100 / 54.  0.100
    libavfilter     2. 77.100 /  2. 77.100
    libswscale      2.  1.100 /  2.  1.100
    libswresample   0. 15.100 /  0. 15.100
    libpostproc    52.  0.100 / 52.  0.100
  [wav @ 0x13d08100] max_analyze_duration 5000000 reached at 5120000
  Guessed Channel Layout for  Input Stream #0.0 : mono
  Input #0, wav, from '10001.wav':
    Duration: 00:00:07.91, bitrate: 64 kb/s
      Stream #0:0: Audio: pcm_alaw ([6][0][0][0] / 0x0006), 8000 Hz, mono, s16, 64 kb/s
  [wav @ 0x13d08980] max_analyze_duration 5000000 reached at 5120000
  Guessed Channel Layout for  Input Stream #1.0 : mono
  Input #1, wav, from '201.wav':
    Duration: 00:00:11.88, bitrate: 128 kb/s
      Stream #1:0: Audio: pcm_s16le ([1][0][0][0] / 0x0001), 8000 Hz, mono, s16, 128 kb/s
  [abuffersink @ 0x13d268c0] No opaque field provided, an AVABufferSinkParams struct is required
  </code></pre>
  
  <p>Can anyone can help me? Thanks!</p>