title: Concatenating audio files with ffmpeg results in a wrong total duration
body: <p>With &quot;wrong total duration&quot; I mean a total duration different from the sum of individual duration of audio files.</p>
  <p>sum_duration_files !=  duration( concatenation of files )</p>
  <p>In particular I am concatenating 2 OGG audio files with this command</p>
  <pre><code>ffmpeg -safe 0 -loglevel quiet \
    -f concat -segment_time_metadata 1 -i {m3u_file_name} \
    -vf select=concatdec_select \
    -af aselect=concatdec_select,aresample=async=1 \
    {ogg_file_name}
  </code></pre>
  <p>And I get the following</p>
  <pre><code># Output of:  ffprobe &lt;FILE&gt;.ogg
  
  
  ======== files_in 
  
  Input #0, ogg, from 'f1.ogg':
    Duration: 00:00:04.32, start: 0.000000, bitrate: 28 kb/s
      Stream #0:0: Audio: opus, 48000 Hz, mono, fltp
  
  
  Input #0, ogg, from 'f2.ogg':
    Duration: 00:00:00.70, start: 0.000000, bitrate: 68 kb/s
      Stream #0:0: Audio: vorbis, 44100 Hz, mono, fltp, 160 kb/s
      Metadata:
        ENCODER         : Lavc57.107.100 libvorbis
  </code></pre>
  <p>Note durations:  4.32 and 0.7 sec</p>
  <p>And this is the output file.</p>
  <pre><code>========== files out (concatenate of files_in)
  
  Input #0, ogg, from 'f_concat_v1.ogg':
    Duration: 00:00:04.61, start: 0.000000, bitrate: 61 kb/s
      Stream #0:0: Audio: vorbis, 48000 Hz, mono, fltp, 80 kb/s
      Metadata:
        ENCODER         : Lavc57.107.100 libvorbis
  </code></pre>
  <p>Duration: 4.61 sec</p>
  <p>As 4.61 sec  !=  4.32 + 0.7 sec  I have a problem.</p>