$ffmpeg -i output.mp4 -i coins.mp3 -codec copy output.mp4
ffmpeg version 3.0.2-2~bpo8+1 Copyright (c) 2000-2016 the FFmpeg developers
  built with gcc 4.9.2 (Debian 4.9.2-10)
  configuration: --prefix=/usr --extra-version='2~bpo8+1' --toolchain=hardened --libdir=/usr/lib/x86_64-linux-gnu --incdir=/usr/include/x86_64-linux-gnu --cc=cc --cxx=g++ --enable-gpl --enable-shared --disable-stripping --disable-decoder=libopenjpeg --disable-decoder=libschroedinger --enable-avresample --enable-avisynth --enable-gnutls --enable-ladspa --enable-libass --enable-libbluray --enable-libbs2b --enable-libcaca --enable-libcdio --enable-libflite --enable-libfontconfig --enable-libfreetype --enable-libfribidi --enable-libgme --enable-libgsm --enable-libmodplug --enable-libmp3lame --enable-libopenjpeg --enable-libopus --enable-libpulse --enable-librubberband --enable-librtmp --enable-libschroedinger --enable-libshine --enable-libsnappy --enable-libsoxr --enable-libspeex --enable-libssh --enable-libtheora --enable-libtwolame --enable-libvorbis --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx265 --enable-libxvid --enable-libzvbi --enable-openal --enable-opengl --enable-x11grab --enable-libdc1394 --enable-libiec61883 --enable-libzmq --enable-frei0r --enable-chromaprint --enable-libx264
  libavutil      55. 17.103 / 55. 17.103
  libavcodec     57. 24.102 / 57. 24.102
  libavformat    57. 25.100 / 57. 25.100
  libavdevice    57.  0.101 / 57.  0.101
  libavfilter     6. 31.100 /  6. 31.100
  libavresample   3.  0.  0 /  3.  0.  0
  libswscale      4.  0.100 /  4.  0.100
  libswresample   2.  0.101 /  2.  0.101
  libpostproc    54.  0.100 / 54.  0.100
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'output.mp4':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2avc1mp41
    encoder         : Lavf57.25.100
  Duration: 00:00:00.70, start: 0.000000, bitrate: 234 kb/s
    Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 154x154, 223 kb/s, 10 fps, 10 tbr, 10240 tbn, 20 tbc (default)
    Metadata:
      handler_name    : VideoHandler
[mp3 @ 0x13d0280] Skipping 0 bytes of junk at 1164.
Input #1, mp3, from 'coins.mp3':
  Duration: 00:00:03.81, start: 0.000000, bitrate: 90 kb/s
    Stream #1:0: Audio: mp3, 22050 Hz, mono, s16p, 88 kb/s
File 'output.mp4' already exists. Overwrite ? [y/N] y
Output #0, mp4, to 'output.mp4':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2avc1mp41
    encoder         : Lavf57.25.100
    Stream #0:0(und): Video: h264 ([33][0][0][0] / 0x0021), yuv420p, 154x154, q=2-31, 223 kb/s, 10 fps, 10 tbr, 10240 tbn, 10240 tbc (default)
    Metadata:
      handler_name    : VideoHandler
    Stream #0:1: Audio: mp3 (i[0][0][0] / 0x0069), 22050 Hz, mono, 88 kb/s
Stream mapping:
  Stream #0:0 -> #0:0 (copy)
  Stream #1:0 -> #0:1 (copy)
Press [q] to stop, [?] for help
frame=    7 fps=0.0 q=-1.0 Lsize=      62kB time=00:00:03.84 bitrate= 132.7kbits/s speed=2.4e+03x
video:19kB audio:41kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 3.307876%