ffmpeg -i La_Vestale-704x400-832k.flv -i lavestaleuff.ac3 -vcodec copy -acodec copy -map 0:v -map 1:a la-vestale-sync.flv
ffmpeg version 3.2.4 Copyright (c) 2000-2017 the FFmpeg developers
  built with gcc 6.2.0 (Ubuntu 6.2.0-5ubuntu12) 20161005
  configuration: --enable-gpl --enable-shared --enable-ffplay --enable-libx264
  libavutil      55. 34.101 / 55. 34.101
  libavcodec     57. 64.101 / 57. 64.101
  libavformat    57. 56.101 / 57. 56.101
  libavdevice    57.  1.100 / 57.  1.100
  libavfilter     6. 65.100 /  6. 65.100
  libswscale      4.  2.100 /  4.  2.100
  libswresample   2.  3.100 /  2.  3.100
  libpostproc    54.  1.100 / 54.  1.100
[flv @ 0x55f82354ee60] video stream discovered after head already parsed
[flv @ 0x55f82354ee60] audio stream discovered after head already parsed
Input #0, flv, from 'La_Vestale-704x400-832k.flv':
  Metadata:
    moovPosition    : 1011616864
    avcprofile      : 77
    avclevel        : 31
    aacaot          : 2
    videoframerate  : 25
    audiochannels   : 2
    length          : 450932736
    timescale       : 48000
    sampletype      : mp4a
  Duration: 02:36:34.44, start: 0.000000, bitrate: 871 kb/s
    Stream #0:0: Video: h264 (Main), yuv420p(progressive), 704x396, 25 fps, 25 tbr, 1k tbn, 50 tbc
    Stream #0:1: Audio: aac (LC), 48000 Hz, stereo, fltp
[ac3 @ 0x55f8235be3c0] Estimating duration from bitrate, this may be inaccurate
Input #1, ac3, from 'lavestaleuff.ac3':
  Duration: 02:36:33.86, start: 0.000000, bitrate: 320 kb/s
    Stream #1:0: Audio: ac3, 48000 Hz, stereo, fltp, 320 kb/s
File 'la-vestale-sync.flv' already exists. Overwrite ? [y/N] y
[flv @ 0x55f823606ea0] FLV does not support sample rate 48000, choose from (44100, 22050, 11025)
[flv @ 0x55f823606ea0] Audio codec ac3 not compatible with flv
Could not write header for output file #0 (incorrect codec parameters ?): Function not implementedStream mapping:
  Stream #0:0 -> #0:0 (copy)
  Stream #1:0 -> #0:1 (copy)
    Last message repeated 1 times