title: FFMpeg - add background music
body: <p>here is what i have:
  input1.avi - video that contain sounds.
  input2.avi - video that doesn't contain sounds.
  music.mp3 - audio file.</p>
  
  <p>i want to add background music(music.mp3 file) to the video.</p>
  
  <pre><code>C:\input1.avi -i C:\music.mp3 -shortest -c:v copy -c:a copy C:\output1.avi
  </code></pre>
  
  <p>then output1.avi is the same as input1 - movie with sounds but without the background music (music.mp3)</p>
  
  <p>when i try to use the other file (video without sounds):</p>
  
  <pre><code>C:\input2.avi -i C:\music.mp3 -shortest -c:v copy -c:a copy C:\output2.avi
  </code></pre>
  
  <p>then output2.avi is the same as input2 + it have the background music.</p>
  
  <p>so why input1 does not contain the background music???
  and how can i decrease or increase the volume of music.mp3 file?</p>
  
  <p>thanks.</p>
  
  <hr>
  
  <p>console output:</p>
  
  <pre><code>C:\motionbee\ffmpeg\bin&gt;ffmpeg -i C:\input.avi
  -i C:\music.mp3 -shortest -c:v copy -filter_
  complex "[0:a]aformat=fltp:44100:stereo[0a];[1]aformat=fltp:44100:stereo,volume=
  1.5[1a];[0a][1a]amix[a]" -map 0:v -map "[a]" -ac 2 C:\output1.avi
  ffmpeg version N-78949-g6f5048f Copyright (c) 2000-2016 the FFmpeg developers
    built with gcc 5.3.0 (GCC)
    configuration: --enable-gpl --enable-version3 --disable-w32threads --enable-av
  isynth --enable-bzlib --enable-fontconfig --enable-frei0r --enable-gnutls --enab
  le-iconv --enable-libass --enable-libbluray --enable-libbs2b --enable-libcaca --
  enable-libdcadec --enable-libfreetype --enable-libgme --enable-libgsm --enable-l
  ibilbc --enable-libmodplug --enable-libmfx --enable-libmp3lame --enable-libopenc
  ore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-libopus --ena
  ble-librtmp --enable-libschroedinger --enable-libsoxr --enable-libspeex --enable
  -libtheora --enable-libtwolame --enable-libvidstab --enable-libvo-amrwbenc --ena
  ble-libvorbis --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx
  264 --enable-libx265 --enable-libxavs --enable-libxvid --enable-libzimg --enable
  -lzma --enable-decklink --enable-zlib
    libavutil      55. 19.100 / 55. 19.100
    libavcodec     57. 27.101 / 57. 27.101
    libavformat    57. 28.100 / 57. 28.100
    libavdevice    57.  0.101 / 57.  0.101
    libavfilter     6. 39.100 /  6. 39.100
    libswscale      4.  0.100 /  4.  0.100
    libswresample   2.  0.101 /  2.  0.101
    libpostproc    54.  0.100 / 54.  0.100
  Input #0, avi, from 'C:\input.avi':
    Metadata:
      encoder         : Lavf57.28.100
    Duration: 00:02:05.76, start: 0.000000, bitrate: 450 kb/s
      Stream #0:0: Video: mpeg4 (Simple Profile) (XVID / 0x44495658), yuv420p, 720
  x480 [SAR 1:1 DAR 3:2], 440 kb/s, 25 fps, 25 tbr, 25 tbn, 25 tbc
      Stream #0:1: Audio: mp3 (U[0][0][0] / 0x0055), 44100 Hz, stereo, s16p, 128 k
  b/s
  [mp3 @ 00000000005c8020] Skipping 0 bytes of junk at 32370.
  Input #1, mp3, from 'C:\music.mp3':
    Metadata:
      title           : Broadcast News Package - News Intro
      artist          : After Effects News Template
    Duration: 00:01:57.89, start: 0.025057, bitrate: 194 kb/s
      Stream #1:0: Audio: mp3, 44100 Hz, stereo, s16p, 192 kb/s
      Metadata:
        encoder         : Lavc56.26
  File 'C:\output1.avi' already exists. Overwrite
  ? [y/N] y
  Output #0, avi, to 'C:\output1.avi':
    Metadata:
      ISFT            : Lavf57.28.100
      Stream #0:0: Video: mpeg4 (XVID / 0x44495658), yuv420p, 720x480 [SAR 1:1 DAR
   3:2], q=2-31, 440 kb/s, 25 fps, 25 tbr, 25 tbn, 25 tbc
      Stream #0:1: Audio: mp3 (libmp3lame) (U[0][0][0] / 0x0055), 44100 Hz, stereo
  , fltp (default)
      Metadata:
        encoder         : Lavc57.27.101 libmp3lame
  Stream mapping:
    Stream #0:1 (mp3) -&gt; aformat
    Stream #1:0 (mp3) -&gt; aformat
    Stream #0:0 -&gt; #0:0 (copy)
    amix -&gt; Stream #0:1 (libmp3lame)
  Press [q] to stop, [?] for help
  frame= 3118 fps=0.0 q=-1.0 Lsize=    6917kB time=00:02:05.76 bitrate= 450.6kbits
  /s speed= 867x
  video:6754kB audio:74kB subtitle:0kB other streams:0kB global headers:0kB muxing
   overhead: 1.307944%
  </code></pre>