$ ffprobe my_input_file.m4v

Stream #0:0(und): Video: h264 (Main) (avc1 / 0x31637661), yuv420p(tv, smpte170m/smpte170m/bt709), 718x552 [SAR 64:45 DAR 5744:3105], 1023 kb/s, 25 fps, 25 tbr, 90k tbn, 180k tbc (default)
    Metadata:
      creation_time   : 2013-01-26 10:39:52
      encoder         : JVT/AVC Coding
    Stream #0:1(fra): Audio: ac3 (ac-3 / 0x332D6361), 48000 Hz, 5.1(side), fltp, 384 kb/s (default)
    Metadata:
      creation_time   : 2013-01-26 10:39:52
    Side data:
      audio service type: main
    Stream #0:2(eng): Audio: ac3 (ac-3 / 0x332D6361), 48000 Hz, 5.1(side), fltp, 384 kb/s
    Metadata:
      creation_time   : 2013-01-26 10:39:52
    Side data:
      audio service type: main

$ ffmpeg -i my_input_file.m4v -map 0:2 -c:a copy output_file.ac3

$ ffmpeg -i my_input_file.m4v -map 0:2 output_file.wav

ffmpeg -i my_input_file.ac3 -map_channel 0.0.0 l.wav -map_channel 0.0.1 r.wav -map_channel 0.0.2 c.wav -map_channel 0.0.3 ls.wav -map_channel 0.0.4 rs.wav -map_channel 0.0.5 lfe.wav