Por que não consigo atrasar dois arquivos de áudio?

Question

Gostaria de atrasar apenas UM dos dois arquivos de entrada. Mas, mesmo com um valor de 5, seu deslocamento não está funcionando .... por que?

ffmpeg -itsoffset 5 -i one.m4a -i five.m4a -strict -2 -filter_complex "[0:a][1:a]amerge,pan=stereo:c0<c0+c2:c1<c1+c3[aout]" -map "[aout]" out.m4a

 ffmpeg version 2.3 Copyright (c) 2000-2014 the FFmpeg developers
      built on Jul 16 2014 21:03:06 with llvm-gcc 4.2.1 (LLVM build 2336.11.00)
      configuration: --prefix=/Volumes/tempdisk/sw --as=yasm --enable-gpl --enable-pthreads --disable-ffplay --disable-ffserver --disable-shared --enable-static --enable-libvpx --disable-decoder=libvpx --enable-libmp3lame --enable-libopus --enable-libtheora --enable-libvorbis --enable-libx264 --enable-libx265 --enable-libxvid --enable-zlib --enable-avfilter --enable-fontconfig --enable-libfreetype --enable-libass --enable-libutvideo --enable-filters --enable-postproc --enable-runtime-cpudetect
      libavutil      52. 92.100 / 52. 92.100
      libavcodec     55. 69.100 / 55. 69.100
      libavformat    55. 48.100 / 55. 48.100
      libavdevice    55. 13.102 / 55. 13.102
      libavfilter     4. 11.100 /  4. 11.100
      libswscale      2.  6.100 /  2.  6.100
      libswresample   0. 19.100 /  0. 19.100
      libpostproc    52.  3.100 / 52.  3.100
    Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'one.m4a':
      Metadata:
        major_brand     : M4A 
        minor_version   : 512
        compatible_brands: isomiso2
        creation_time   : 1970-01-01 00:00:00
        encoder         : Lavf52.64.2
      Duration: 00:00:04.41, start: 0.000000, bitrate: 153 kb/s
        Stream #0:0(und): Audio: aac (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 149 kb/s (default)
        Metadata:
          creation_time   : 1970-01-01 00:00:00
          handler_name    : SoundHandler
    Input #1, mov,mp4,m4a,3gp,3g2,mj2, from 'five.m4a':
      Metadata:
        major_brand     : M4A 
        minor_version   : 512
        compatible_brands: isomiso2
        creation_time   : 1970-01-01 00:00:00
        encoder         : Lavf52.64.2
      Duration: 00:00:04.41, start: 0.000000, bitrate: 151 kb/s
        Stream #1:0(und): Audio: aac (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 147 kb/s (default)
        Metadata:
          creation_time   : 1970-01-01 00:00:00
          handler_name    : SoundHandler
    [Parsed_pan_1 @ 0x7f8f63d038e0] This syntax is deprecated. Use '|' to separate the list items.
    [Parsed_amerge_0 @ 0x7f8f63d035a0] No channel layout for input 1
    [Parsed_amerge_0 @ 0x7f8f63d035a0] Input channel layouts overlap: output layout will be determined by the number of distinct input channels
    Output #0, ipod, to 'o.m4a':
      Metadata:
        major_brand     : M4A 
        minor_version   : 512
        compatible_brands: isomiso2
        encoder         : Lavf55.48.100
        Stream #0:0: Audio: aac (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 128 kb/s (default)
        Metadata:
          encoder         : Lavc55.69.100 aac
    Stream mapping:
      Stream #0:0 (aac) -> amerge:in0
      Stream #1:0 (aac) -> amerge:in1
      pan -> Stream #0:0 (aac)
    Press [q] to stop, [?] for help
    size=      70kB time=00:00:06.41 bitrate=  89.7kbits/s    
    video:0kB audio:69kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 2.148568%