ffmpeg -i input.mkv -acodec ac3 -ab 640k -ac 6 -vcodec copy output.mkv
ffmpeg version N-32077-g5f5e37e, Copyright (c) 2000-2011 the FFmpeg developers
  built on Aug 24 2011 17:01:01 with gcc 4.4.5
  configuration: --enable-gpl --enable-libfaac --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libtheora --enable-libvorbis --enable-libx264 --enable-libxvid --enable-nonfree --enable-postproc --enable-version3 --enable-x11grab
  libavutil    51. 13. 0 / 51. 13. 0
  libavcodec   53. 12. 0 / 53. 12. 0
  libavformat  53.  9. 0 / 53.  9. 0
  libavdevice  53.  3. 0 / 53.  3. 0
  libavfilter   2. 34. 2 /  2. 34. 2
  libswscale    2.  0. 0 /  2.  0. 0
  libpostproc  51.  2. 0 / 51.  2. 0
[matroska,webm @ 0x14d9c60] Estimating duration from bitrate, this may be inaccurate
Input #0, matroska,webm, from 'input.mkv':
  Metadata:
    title           : INPUT
  Duration: 01:45:58.22, start: 0.000000, bitrate: 1536 kb/s
    Stream #0.0: Video: h264 (High), yuv420p, 1920x1080, SAR 1:1 DAR 16:9, 23.98 fps, 23.98 tbr, 1k tbn, 47.95 tbc (default)
    Stream #0.1(eng): Audio: dts (DTS), 48000 Hz, 5.1, s16, 1536 kb/s (default)
Incompatible sample format 's16' for codec 'ac3', auto-selecting format 'flt'
[ac3 @ 0x14d3fa0] channel_layout not specified
[ac3 @ 0x14d3fa0] No channel layout specified. The encoder will guess the layout, but it might be incorrect.
Output #0, matroska, to 'output.mkv':
  Metadata:
    title           : INPUT
    encoder         : Lavf53.9.0
    Stream #0.0: Video: h264 (H264 / 0x34363248), yuv420p, 1920x1080 [SAR 1:1 DAR 16:9], q=2-31, 1k tbn, 1k tbc (default)
    Stream #0.1(eng): Audio: ac3 ([0] [0][0] / 0x2000), 48000 Hz, 5.1, flt, 640 kb/s (default)
Stream mapping:
  Stream #0.0 -> #0.0: copy
  Stream #0.1 -> #0.1: dca -> ac3
Press [q] to stop, [?] for help
[matroska @ 0x14dd7c0] pts < dts in stream 0
av_interleaved_write_frame(): Invalid argument