Video
MPEG Streamclip,
Warning: I know nothing about video editing or sound.
Problems
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Sound from mp4 file low
On Mac I used MPEG Streamclip to export mp4 without sound by
deleting sound track then exporting
and export the sound as an mp3.
Edited the sound with Audacity > selected all of the track >
Effect > Amplify > capped and saved sound file as mp3.
Then used Avidemux to merge the video and sound. Had problems
with audio setting in two places - in the left panel under audio
and menu Audio > Main track > External MP3 > select mp3 file.
> "Save" the file exports new file, then test.