[agustin@agustin Family.Guy.S16E13.720p.HDTV.x264-AVS[rarbg]]$ ffmpeg -i Family.Guy.S16E13.720p.HDTV.x264-AVS.mkv -vcodec copy -acodec mp3 Family.Guy.S16E13.720p.HDTV.x264-AVS.mp4
ffmpeg version 3.4.2 Copyright (c) 2000-2018 the FFmpeg developers
  built with gcc 7.3.0 (GCC)
  configuration: --prefix=/usr --disable-debug --disable-static --disable-stripping --enable-avisynth --enable-avresample --enable-fontconfig --enable-gmp --enable-gnutls --enable-gpl --enable-ladspa --enable-libass --enable-libbluray --enable-libfreetype --enable-libfribidi --enable-libgsm --enable-libiec61883 --enable-libmodplug --enable-libmp3lame --enable-libopencore_amrnb --enable-libopencore_amrwb --enable-libopenjpeg --enable-libopus --enable-libpulse --enable-libsoxr --enable-libspeex --enable-libssh --enable-libtheora --enable-libv4l2 --enable-libvidstab --enable-libvorbis --enable-libvpx --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxcb --enable-libxml2 --enable-libxvid --enable-shared --enable-version3 --enable-omx
  libavutil      55. 78.100 / 55. 78.100
  libavcodec     57.107.100 / 57.107.100
  libavformat    57. 83.100 / 57. 83.100
  libavdevice    57. 10.100 / 57. 10.100
  libavfilter     6.107.100 /  6.107.100
  libavresample   3.  7.  0 /  3.  7.  0
  libswscale      4.  8.100 /  4.  8.100
  libswresample   2.  9.100 /  2.  9.100
  libpostproc    54.  7.100 / 54.  7.100
Input #0, matroska,webm, from 'Family.Guy.S16E13.720p.HDTV.x264-AVS.mkv':
  Metadata:
    encoder         : libx264 + libebml + libmatroska + libfastrar
    creation_time   : 2018-03-26T01:10:14.000000Z
  Duration: 00:20:55.55, start: 0.000000, bitrate: 3412 kb/s
    Stream #0:0: Video: h264 (High), yuv420p(tv, bt709/unknown/unknown, progressive), 1280x720, SAR 1:1 DAR 16:9, 23.98 fps, 23.98 tbr, 1k tbn, 47.95 tbc (default)
    Stream #0:1: Audio: ac3, 48000 Hz, 5.1(side), fltp, 448 kb/s (default)
File 'Family.Guy.S16E13.720p.HDTV.x264-AVS.mp4' already exists. Overwrite ? [y/N] y
Stream mapping:
  Stream #0:0 -> #0:0 (copy)
  Stream #0:1 -> #0:1 (ac3 (native) -> mp3 (libmp3lame))
Press [q] to stop, [?] for help
Output #0, mp4, to 'Family.Guy.S16E13.720p.HDTV.x264-AVS.mp4':
  Metadata:
    encoder         : Lavf57.83.100
    Stream #0:0: Video: h264 (High) (avc1 / 0x31637661), yuv420p(tv, bt709/unknown/unknown, progressive), 1280x720 [SAR 1:1 DAR 16:9], q=2-31, 23.98 fps, 23.98 tbr, 16k tbn, 1k tbc (default)
    Stream #0:1: Audio: mp3 (libmp3lame) (mp4a / 0x6134706D), 48000 Hz, stereo, fltp (default)
    Metadata:
      encoder         : Lavc57.107.100 libmp3lame
Invalid return value 0 for stream protocol time=00:20:48.60 bitrate=3065.3kbits/s speed=40.7x
    Last message repeated 1 times
frame=30103 fps=975 q=-1.0 Lsize=  474543kB time=00:20:55.53 bitrate=3096.3kbits/s speed=40.7x
video:454020kB audio:19618kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.191187%
[agustin@agustin Family.Guy.S16E13.720p.HDTV.x264-AVS[rarbg]]$ ffmpeg -version
ffmpeg version 3.4.2 Copyright (c) 2000-2018 the FFmpeg developers
built with gcc 7.3.0 (GCC)
configuration: --prefix=/usr --disable-debug --disable-static --disable-stripping --enable-avisynth --enable-avresample --enable-fontconfig --enable-gmp --enable-gnutls --enable-gpl --enable-ladspa --enable-libass --enable-libbluray --enable-libfreetype --enable-libfribidi --enable-libgsm --enable-libiec61883 --enable-libmodplug --enable-libmp3lame --enable-libopencore_amrnb --enable-libopencore_amrwb --enable-libopenjpeg --enable-libopus --enable-libpulse --enable-libsoxr --enable-libspeex --enable-libssh --enable-libtheora --enable-libv4l2 --enable-libvidstab --enable-libvorbis --enable-libvpx --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxcb --enable-libxml2 --enable-libxvid --enable-shared --enable-version3 --enable-omx
libavutil      55. 78.100 / 55. 78.100
libavcodec     57.107.100 / 57.107.100
libavformat    57. 83.100 / 57. 83.100
libavdevice    57. 10.100 / 57. 10.100
libavfilter     6.107.100 /  6.107.100
libavresample   3.  7.  0 /  3.  7.  0
libswscale      4.  8.100 /  4.  8.100
libswresample   2.  9.100 /  2.  9.100
libpostproc    54.  7.100 / 54.  7.100