ffmpeg -i movie.mkv -vn -f wav /dev/null
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 @ 0x170e3e0] Increasing reorder buffer to 1
Input #0, matroska,webm, from 'movie.mkv':
  Duration: 00:42:05.05, start: 0.000000, bitrate: 4270 kb/s
    Stream #0:0(eng): Audio: aac, 48000 Hz, stereo, s16 (default)
    Stream #0:1(eng): Video: h264 (Main), yuv420p, 960x720 [SAR 4:3 DAR 16:9], 29.97 fps, 29.97 tbr, 1k tbn, 2k tbc (default)
File '/dev/null' already exists. Overwrite ? [y/N] y
Output #0, wav, to '/dev/null':
  Metadata:
    encoder         : Lavf54.0.100
    Stream #0:0(eng): Audio: pcm_s16le ([1][0][0][0] / 0x0001), 48000 Hz, stereo, s16, 1536 kb/s (default)
Stream mapping:
  Stream #0:0 -> #0:0 (aac -> pcm_s16le)
Press [q] to stop, [?] for help
size=       0kB time=00:42:05.05 bitrate=   0.0kbits/s
video:0kB audio:473448kB global headers:0kB muxing overhead -100.000000%