Something

ffmpeg -i Realisation.mp4 -i One.mp3 -filter_complex "[1:a]volume=-6db,aloop=8:2^31-1[quieter];[0:a][quieter]amix=inputs=2:duration=shortest[a]" -c:v copy -c:a libmp3lame -ac 2 output.mp4 

ffmpeg version N-76045-g97be5d4 Copyright (c) 2000-2015 the FFmpeg developers
  built with gcc 4.8 (Ubuntu 4.8.4-2ubuntu1~14.04)
  configuration: --extra-libs=-ldl --prefix=/opt/ffmpeg --enable-avresample --disable-debug --enable-nonfree --enable-gpl --enable-version3 --enable-libopencore-amrnb --enable-libopencore-amrwb --disable-decoder=amrnb --disable-decoder=amrwb --enable-libpulse --enable-libdcadec --enable-libfreetype --enable-libx264 --enable-libx265 --enable-libfdk-aac --enable-libvorbis --enable-libmp3lame --enable-libopus --enable-libvpx --enable-libspeex --enable-libass --enable-avisynth --enable-libsoxr --enable-libxvid --enable-libvo-aacenc --enable-libvidstab
  libavutil      55.  4.100 / 55.  4.100
  libavcodec     57.  6.100 / 57.  6.100
  libavformat    57.  4.100 / 57.  4.100
  libavdevice    57.  0.100 / 57.  0.100
  libavfilter     6. 11.100 /  6. 11.100
  libavresample   3.  0.  0 /  3.  0.  0
  libswscale      4.  0.100 /  4.  0.100
  libswresample   2.  0.100 /  2.  0.100
  libpostproc    54.  0.100 / 54.  0.100
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'Realisation.mp4':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2avc1mp41
    encoder         : Lavf57.34.101
  Duration: 00:08:36.00, start: 0.000000, bitrate: 1773 kb/s
    Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 1280x720, 1647 kb/s, 30 fps, 30 tbr, 15360 tbn, 60 tbc (default)
    Metadata:
      handler_name    : VideoHandler
    Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 117 kb/s (default)
    Metadata:
      handler_name    : SoundHandler
[mp3 @ 0x2164340] Skipping 0 bytes of junk at 196.
[mp3 @ 0x2164340] Estimating duration from bitrate, this may be inaccurate
Input #1, mp3, from 'One.mp3':
  Metadata:
    title           : Dinner For One
    artist          : Silent Partner
    album           : YouTube Audio Library
    genre           : Rock
    encoder         : Google
  Duration: 00:02:57.97, start: 0.000000, bitrate: 320 kb/s
    Stream #1:0: Audio: mp3, 44100 Hz, stereo, s16p, 320 kb/s
[Parsed_volume_0 @ 0x21d79a0] [Eval @ 0x7fffd38185c0] Invalid chars 'b' at the end of expression '-6db'
[Parsed_volume_0 @ 0x21d79a0] Error when evaluating the volume expression '-6db'
[AVFilterGraph @ 0x21649c0] Error initializing filter 'volume' with args '-6db'
Error initializing complex filters.
Invalid argument