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Short question :

I have taken a video screen recording on my iPhone and imported it to my computer, when i try to compress or share the file it keeps stripping the audio from the video and im just left with a video with no sound. Any ideas why ?

Long questions with background :

I have a screen recording video that I've done on my iPhone 8, iOS 12, showing someone how to do something in the Sonos app, in this video you can hear music playing muffled in the background as i show me changing tracks and can see album covers on my screen.

Ive imported the screen recording to my computer running OSX 10.11 and it works fine. I then trimmed the video in quicktime to remove the beginning, still all working fine. But now ive got a 70MB 2 minute video i cant email.

I get the videos from my iOS device to my OSX device using the dropbox App on the iOS device to upload the video. The file format is Mp4.

Ive tried running it through Handbrake, to compress the video which takes it down to c. 9MB, but for some reason it strips the audio, even through the the audio settings are not set to strip the audio.

Here are the settings im using in Handrake : https://imgur.com/a/GOBwmUc

I then tried to add the 70MB video to my dropbox folder and sharing the video from there, when i create the share link it again has now audio.

I then tried uploading it to youtube as a private video, but again the video has no audio.

Any ideas whats going on here, i would expect Youtube to remove audio tracks if its algorithm detected copyrighted data, but not dropbox, or Handbrake. Is this what is happening ? Or do iOS screen recored videos encode their audio tracks in some odd way that other systems have a hard time processing ?

How can i get around this so that i can share this video with the person i made it for ?

Without knowing which video player you used for playback, here is my solution:
How you transfer the video shouldn't matter, unless you use facebook, as they will compress the video.

First a bit of information about the video:

Apple iOS screen recorder produce AVC video using High@L4 format profile with 2 reference frames (CABAC). The frame rate is variable, but average at ~56--59 fps.
It has 8 bits with colorspace YUV and uses progressive scantype.

Audio is AAC LC (Advanced Audio Codec Low Complexity) with the ID mp4a-40-2 over stereo at normal sampling rate of 44.1kHz.

This is packed in a MPEG-4 container with id mp41/mp42/isom

The setup

My initial file is 3080KB directly downloaded from my iOS device (iphone 6), I used spotify as an example. The result file turned out to be 390KB, which is approx 87.337% compression rate---should be enough for email considering your file size, if not you can turn down Constant Quality even more (say 34--38).

I used Handbrake 1.1.2 64bit. And I test the pre and post videos using VLC 3.0.4 Vetinari.

Here are my settings:

Summary

Dimensions

Not really needed, I've found best results with mod 4 over mod 2.

Filters

No filters, set Deinterlace to "off".

Video

I use CQ of 32, and set encoder preset to veryslow for best compression and quality results.

Audio

I use ACC Passthru, this will just copy the stream of the original file, and since the size of the audio is quite small, this won't do much difference. The difference is in the video.

The video will play with audio on most modern devices and computers.

You are converting an MP4 container with AAC audio but unknown(?) video codec
to an H.264 container with x264 video codec and a not-shown audio codec.

My guess would be on a codec problem with Handbrake.
As Handbrake uses the installed codecs, some codec used by the iPhone
is missing or badly supported on your computer.

I would suggest doing the conversion using another converter.
I recommend using
VLC for Mac OSX,
which comes with its own codecs and is one of the world's best
player/converter/streamer.

For more information see the article
Convert Video File Format using VLC Media Player.

As the reason for this exercise is the sending of the video by email,
solutions exist for that purpose using free services, where 70 MB is not counted
as very big. Some examples, in no particular order, are:
We Transfer,
Jumpshare and
Securely Send.

If the above does not solve the problem,
you could post your 5 MB sample for further analysis.