root@localhost:/media/c562c662-acad-4cd2-9a3a-e5b30fc367e9/downloads# ffmpeg -i input.mkv -acodec copy -vcodec copy output.flv
ffmpeg version 0.9.1.git Copyright (c) 2000-2012 the FFmpeg developers
  built on Mar 11 2012 03:05:56 with gcc 4.4.5
  configuration:
  libavutil      51. 42.100 / 51. 42.100
  libavcodec     54. 10.100 / 54. 10.100
  libavformat    54.  2.100 / 54.  2.100
  libavdevice    53.  4.100 / 53.  4.100
  libavfilter     2. 64.101 /  2. 64.101
  libswscale      2.  1.100 /  2.  1.100
  libswresample   0.  7.100 /  0.  7.100
Input #0, matroska,webm, from 'input.mkv':
  Duration: 00:55:43.54, start: 0.000000, bitrate: 4008 kb/s
    Stream #0:0(eng): Video: h264 (High), yuv420p, 1280x720, SAR 1:1 DAR 16:9, 23.98 fps, 23.98 tbr, 1k tbn, 47.95 tbc (default)
    Stream #0:1(eng): Audio: ac3, 48000 Hz, 5.1(side), s16, 384 kb/s (default)
[NULL @ 0x18986e0] flv does not support that sample rate, choose from (44100, 22050, 11025).
Output #0, flv, to 'output.flv':
  Metadata:
    encoder         : Lavf54.2.100
    Stream #0:0(eng): Video: h264 ([7][0][0][0] / 0x0007), yuv420p, 1280x720 [SAR 1:1 DAR 16:9], q=2-31, 23.98 fps, 1k tbn, 1k tbc (default)
    Stream #0:1(eng): Audio: ac3, 48000 Hz, 5.1(side), 384 kb/s (default)
Stream mapping:
  Stream #0:0 -> #0:0 (copy)
  Stream #0:1 -> #0:1 (copy)
Could not write header for output file #0 (incorrect codec parameters ?)
root@localhost:/media/c562c662-acad-4cd2-9a3a-e5b30fc367e9/downloads#