$ ffmpeg -i DSCN5008.AVI -acodec copy -vn -y a.aac
FFmpeg version SVN-r0.5.9-4:0.5.9-0ubuntu0.10.04.3, Copyright (c) 2000-2009 Fabrice Bellard, et al.
  configuration: --extra-version=4:0.5.9-0ubuntu0.10.04.3 --prefix=/usr --enable-avfilter --enable-avfilter-lavf --enable-vdpau --enable-bzlib --enable-libgsm --enable-libschroedinger --enable-libspeex --enable-libtheora --enable-libvorbis --enable-pthreads --enable-zlib --disable-stripping --disable-vhook --enable-runtime-cpudetect --enable-gpl --enable-postproc --enable-swscale --enable-x11grab --enable-libdc1394 --enable-shared --disable-static
  libavutil     49.15. 0 / 49.15. 0
  libavcodec    52.20. 1 / 52.20. 1
  libavformat   52.31. 0 / 52.31. 0
  libavdevice   52. 1. 0 / 52. 1. 0
  libavfilter    0. 4. 0 /  0. 4. 0
  libswscale     0. 7. 1 /  0. 7. 1
  libpostproc   51. 2. 0 / 51. 2. 0
  built on Jan 24 2013 19:42:21, gcc: 4.4.3
Input #0, avi, from 'DSCN5008.AVI':
  Duration: 00:00:21.43, start: 0.000000, bitrate: 32094 kb/s
    Stream #0.0: Video: mjpeg, yuvj422p, 1280x720, 30 tbr, 30 tbn, 30 tbc
    Stream #0.1: Audio: pcm_s16le, 22050 Hz, mono, s16, 352 kb/s
Output #0, adts, to 'a.aac':
    Stream #0.0: Audio: pcm_s16le, 22050 Hz, mono, s16, 352 kb/s
Stream mapping:
  Stream #0.1 -> #0.0
Press [q] to stop encoding
size=     923kB time=21.42 bitrate= 352.8kbits/s
video:0kB audio:923kB global headers:0kB muxing overhead 0.000000%