Generated videos have no sound in Apple QuickTime Player

[glynnbird]

If I generate a video montage with Stitcheroo and double click it on my Mac it opens in QuickTime Player but has no sound. If I drag the file onto Google Chrome, the sound plays ok.

Extra data:

ffprobe 'final(1).mp4'
ffprobe version 4.2.3 Copyright (c) 2007-2020 the FFmpeg developers
  built with Apple clang version 11.0.3 (clang-1103.0.32.59)
  configuration: --prefix=/usr/local/Cellar/ffmpeg/4.2.3_1 --enable-shared --enable-pthreads --enable-version3 --enable-avresample --cc=clang --host-cflags=-fno-stack-check --host-ldflags= --enable-ffplay --enable-gnutls --enable-gpl --enable-libaom --enable-libbluray --enable-libdav1d --enable-libmp3lame --enable-libopus --enable-librubberband --enable-libsnappy --enable-libsrt --enable-libtesseract --enable-libtheora --enable-libvidstab --enable-libvorbis --enable-libvpx --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxvid --enable-lzma --enable-libfontconfig --enable-libfreetype --enable-frei0r --enable-libass --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-librtmp --enable-libspeex --enable-libsoxr --enable-videotoolbox --disable-libjack --disable-indev=jack
  libavutil      56. 31.100 / 56. 31.100
  libavcodec     58. 54.100 / 58. 54.100
  libavformat    58. 29.100 / 58. 29.100
  libavdevice    58.  8.100 / 58.  8.100
  libavfilter     7. 57.100 /  7. 57.100
  libavresample   4.  0.  0 /  4.  0.  0
  libswscale      5.  5.100 /  5.  5.100
  libswresample   3.  5.100 /  3.  5.100
  libpostproc    55.  5.100 / 55.  5.100
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'final(1).mp4':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2avc1mp41
    encoder         : Lavf58.29.100
  Duration: 00:04:16.95, start: 0.000000, bitrate: 3578 kb/s
    Stream #0:0(und): Audio: mp3 (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 128 kb/s (default)
    Metadata:
      handler_name    : SoundHandler
    Stream #0:1(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 1920x1080, 3444 kb/s, 25 fps, 25 tbr, 12800 tbn, 50 tbc (default)
    Metadata:
      handler_name    : VideoHandler

[itsmedonttell]

ffmpeg is transcoding the mixed audio as mp3 but it's inside an mp4a container. Quicktime could overlook this, but I guess it's not the type. It won't happen anymore if you remove this part from your ffmpeg command: '-c:a mp3'. Alternatively, you can also choose to be explicit: '-c:a m4a'. You can even modify or 'fix' any videos made this way previously by re-transcoding their audio streams.