> ffmpeg -i vid.mp4 -i rec.mp3 -filter_complex "[0:a][1:a]amerge[a]" -strict experimental -c:a aac -map 0:v -map "[a]" -ar 48000 -ab 128k -c:v copy -f mp4 out.mp4 -y

ffmpeg version N-61286-gdbc3e11 Copyright (c) 2000-2014 the FFmpeg developers
  built on Mar 11 2014 22:01:37 with gcc 4.8.2 (GCC)
  configuration: --enable-gpl --enable-version3 --disable-w32threads --enable-avisynth --enable-bzlib --enable-fontconfig --enable-frei0r --enable-gnutls --enable-iconv --enable-libass --enable-libbluray --enable-libcaca --enable-libfreetype --enable-libgsm --enable-libilbc --enable-libmodplug --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-libopus --enable-librtmp --enable-libschroedinger --enable-libsoxr --enable-libspeex --enable-libtheora --enable-libtwolame --enable-libvidstab --enable-libvo-aacenc --enable-libvo-amrwbenc --enable-libvorbis --enable-libvpx --enable-libwavpack --enable-libx264 --enable-libx265 --enable-libxavs --enable-libxvid --enable-zlib
  libavutil      52. 66.101 / 52. 66.101
  libavcodec     55. 52.102 / 55. 52.102
  libavformat    55. 34.100 / 55. 34.100
  libavdevice    55. 11.100 / 55. 11.100
  libavfilter     4.  3.100 /  4.  3.100
  libswscale      2.  5.101 /  2.  5.101
  libswresample   0. 18.100 /  0. 18.100
  libpostproc    52.  3.100 / 52.  3.100
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'vid.mp4':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2avc1mp41
    encoder         : Lavf55.34.100
  Duration: 00:00:20.07, start: 0.033333, bitrate: 214 kb/s
    Stream #0:0(eng): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 480x270 [SAR 1:1 DAR 16:9], 80 kb/s, 25 fps, 25 tbr, 12800 tbn, 50 tbc (default)
    Metadata:
      handler_name    : VideoHandler
    Stream #0:1(eng): Audio: aac (mp4a / 0x6134706D), 48000 Hz, mono, fltp, 128 kb/s (default)
    Metadata:
      handler_name    : SoundHandler
[mp3 @ 02c6c2a0] Estimating duration from bitrate, this may be inaccurate
Input #1, mp3, from 'rec.mp3':
  Duration: 00:00:50.31, start: 0.000000, bitrate: 127 kb/s
    Stream #1:0: Audio: mp3, 44100 Hz, mono, s16p, 128 kb/s
[Parsed_amerge_0 @ 02d514e0] No channel layout for input 1
[Parsed_amerge_0 @ 02d514e0] Input channel layouts overlap: output layout will be determined by the number of distinct input channels
Output #0, mp4, to 'out.mp4':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2avc1mp41
    encoder         : Lavf55.34.100
    Stream #0:0(eng): Video: h264 ([33][0][0][0] / 0x0021), yuv420p, 480x270 [SAR 1:1 DAR 16:9], q=2-31, 80 kb/s, 25 fps, 12800 tbn, 12800 tbc (default)
    Metadata:
      handler_name    : VideoHandler
    Stream #0:1: Audio: aac ([64][0][0][0] / 0x0040), 48000 Hz, stereo, fltp, 128 kb/s (default)
Stream mapping:
  Stream #0:1 (aac) -> amerge:in0
  Stream #1:0 (mp3) -> amerge:in1
  Stream #0:0 -> #0:0 (copy)
  amerge -> Stream #0:1 (aac)
Press [q] to stop, [?] for help
[mp3 @ 02c6f960] overread, skip -6 enddists: -4 -4
frame=  501 fps=227 q=-1.0 Lsize=     527kB time=00:00:20.07 bitrate= 215.0kbits/s
video:196kB audio:315kB subtitle:0 data:0 global headers:0kB muxing overhead 3.170177%
















ffmpeg -i vid.mp4 -i rec.mp3 -filter_complex "[0:a][1:a]amerge[a]" -map 0:v -map "[a]" -c:v copy out.mp4 -y