title: ffmpeg how to select the audio channel with the biggest bps?
body: <p>I have mp4 file with below format,
  <code>
  $ ffprobe -i 70_1.mp4 -hide_banner
  Input #0, mov,mp4,m4a,3gp,3g2,mj2, from '70_1.mp4':
    Metadata:
      major_brand     : qt
      minor_version   : 0
      compatible_brands: qt
      creation_time   : 2018-10-30T08:57:38.000000Z
    Duration: 00:13:55.76, start: 0.000000, bitrate: 2246 kb/s
      Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuvj420p(pc, smpte170m/bt709/unknown, progressive), 1440x1920, 2118 kb/s, 58.75 fps, 60 tbr, 600 tbn, 1200 tbc (default)
      Metadata:
        creation_time   : 2018-10-30T08:57:38.000000Z
        handler_name    : Core Media Video
        encoder         : 'avc1'
      Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 2 kb/s (default)
      Metadata:
        creation_time   : 2018-10-30T08:57:38.000000Z
        handler_name    : Core Media Audio
      Stream #0:2(und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 119 kb/s (default)
      Metadata:
        creation_time   : 2018-10-30T08:57:38.000000Z
        handler_name    : Core Media Audio
  </code>
  As the auto channel is special, there is no sound when play in the browser.</p>
  
  <p>So I use ffmpeg to transfer it with <code>ffmpeg -i 70_1.mp4 -crf 30 -b:a 23k -b:v 96k -threads 4 -map 0:0 -map 0:2 -y ooo.mp4</code>, the generated mp4 file can play in the browser.
  <code>
  $ ffprobe.exe ooo.mp4 -hide_banner
  Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'ooo.mp4':
    Metadata:
      major_brand     : isom
      minor_version   : 512
      compatible_brands: isomiso2avc1mp41
      encoder         : Lavf58.17.101
    Duration: 00:13:55.79, start: 0.000000, bitrate: 246 kb/s
      Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuvj420p(pc), 1440x1920, 209 kb/s, 60 fps, 60 tbr, 15360 tbn, 120 tbc (default)
      Metadata:
        handler_name    : Core Media Video
      Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 23 kb/s (default)
      Metadata:
        handler_name    : Core Media Audio
  </code></p>
  
  <p>But there is one issue, I need to process mp4 file with many formats, some with 1 audio channel, some with 2, So I want to know how to select the audio channel with the biggest bps.</p>