ffmpeg -i movie.mkv -vcodec copy -acodec libfaac -ac 6 -ab 384k movie.mp4
ffmpeg version git-2012-02-05-8162c6f Copyright (c) 2000-2012 the FFmpeg developers
  built on Feb  5 2012 14:33:50 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-nonfree --enable-postproc --enable-version3 --enable-x11grab --enable-libvpx
  libavutil      51. 37.100 / 51. 37.100
  libavcodec     54.  0.102 / 54.  0.102
  libavformat    54.  0.100 / 54.  0.100
  libavdevice    53.  4.100 / 53.  4.100
  libavfilter     2. 61.100 /  2. 61.100
  libswscale      2.  1.100 /  2.  1.100
  libswresample   0.  6.100 /  0.  6.100
  libpostproc    52.  0.100 / 52.  0.100
[h264 @ 0x170dc80] Increasing reorder buffer to 1
Input #0, matroska,webm, from 'movie.mkv':
  Duration: 00:42:14.88, start: 0.000000, bitrate: 4547 kb/s
    Stream #0:0(eng): Video: h264 (Main), yuv420p, 960x720 [SAR 4:3 DAR 16:9], 29.97 fps, 29.97 tbr, 1k tbn, 2k tbc (default)
    Stream #0:1(eng): Audio: ac3, 48000 Hz, 5.1(side), s16, 384 kb/s (default)
File 'movie.mp4' already exists. Overwrite ? [y/N] y
Output #0, mp4, to 'movie.mp4':
  Metadata:
    encoder         : Lavf54.0.100
    Stream #0:0(eng): Video: h264 (![0][0][0] / 0x0021), yuv420p, 960x720 [SAR 4:3 DAR 16:9], q=2-31, 29.97 fps, 1k tbn, 1k tbc (default)
    Stream #0:1(eng): Audio: aac (@[0][0][0] / 0x0040), 48000 Hz, 6 channels, s16, 384 kb/s (default)
Stream mapping:
  Stream #0:0 -> #0:0 (copy)
  Stream #0:1 -> #0:1 (ac3 -> libfaac)
Press [q] to stop, [?] for help
[mp4 @ 0x18560e0] Application provided invalid, non monotonically increasing dts to muxer in stream 0: 67 >= 33
av_interleaved_write_frame(): Invalid argument