title: avconv (ffmpeg): correct audio codec
body: <p>I have an error when tryying to convert video from flash recorder to mp4:</p>
  
  <pre><code>ubuntu@media:/data/media/failed$ avconv -i videoStream_1356794393712_805.flv -acodec aac  videoStream_1356794393712_805.mp4
  avconv version 0.8.4-4:0.8.4-0ubuntu0.12.04.1, Copyright (c) 2000-2012 the Libav developers
    built on Nov  6 2012 16:51:33 with gcc 4.6.3
  [libspeex @ 0x1091500] Missing Speex header, assuming defaults.
  [flv @ 0x108d7a0] Estimating duration from bitrate, this may be inaccurate
  Input #0, flv, from 'videoStream_1356794393712_805.flv':
    Metadata:
      creationdate    : Sat Dec 29 15:20:12
    Duration: 00:00:04.39, start: 0.000000, bitrate: N/A
      Stream #0.0: Video: h264 (Main), yuv420p, 640x480 [PAR 1:1 DAR 4:3], 30 tbr, 1k tbn, 60 tbc
      Stream #0.1: Audio: libspeex, 16000 Hz, 1 channels, s16
  [buffer @ 0x1093a00] w:640 h:480 pixfmt:yuv420p
  [mpeg4 @ 0x1094ce0] removing common factors from framerate
  [aac @ 0x108d120] Too many bits per frame requested
  Output #0, mp4, to 'videoStream_1356794393712_805.mp4':
    Metadata:
      creationdate    : Sat Dec 29 15:20:12
      Stream #0.0: Video: mpeg4, yuv420p, 640x480 [PAR 1:1 DAR 4:3], q=2-31, 200 kb/s, 90k tbn, 30 tbc
      Stream #0.1: Audio: aac, 16000 Hz, 1 channels, s16, 200 kb/s
  Stream mapping:
    Stream #0:0 -&gt; #0:0 (h264 -&gt; mpeg4)
    Stream #0:1 -&gt; #0:1 (libspeex -&gt; aac)
  Error while opening encoder for output stream #0:1 - maybe incorrect parameters such as bit_rate, rate, width or height
  ubuntu@media:/data/media/failed$
  </code></pre>
  
  <p>Ubuntu 12.04 LTS, default ffmpeg/avconv.<br>
  How can I solve this problem?</p>
  
  <p>ffprobe log:</p>
  
  <pre><code>ubuntu@media:/data/media/failed$ ffprobe videoStream_1356794393712_805.flv
  avprobe version 0.8.4-4:0.8.4-0ubuntu0.12.04.1, Copyright (c) 2007-2012 the Libav developers
    built on Nov  6 2012 16:51:33 with gcc 4.6.3
  [libspeex @ 0x1c27500] Missing Speex header, assuming defaults.
  [flv @ 0x1c237a0] Estimating duration from bitrate, this may be inaccurate
  Input #0, flv, from 'videoStream_1356794393712_805.flv':
    Metadata:
      creationdate    : Sat Dec 29 15:20:12
    Duration: 00:00:04.39, start: 0.000000, bitrate: N/A
      Stream #0.0: Video: h264 (Main), yuv420p, 640x480 [PAR 1:1 DAR 4:3], 30 tbr, 1k tbn, 60 tbc
      Stream #0.1: Audio: libspeex, 16000 Hz, 1 channels, s16
  [libspeex @ 0x1c27500] Missing Speex header, assuming defaults.
  ubuntu@media:/data/media/failed$
  </code></pre>