Hi
I guess that is the video size not the sound file.
As Steve says this is some task ,
Do you still have the sound available from a separate track in the original videos,
Was it ever available post video production as an audio recording or did the camera embed it in the video.
Or has it already been rendered into the composite video MP4 file or what other movie format you use.

Worse case would be to record the sound off the clip then work on the audio do not change its time of cut in any way . but you should be able to compress .
You would then have to make a new Video from your original with the sound muted,
Then overdub your new Audio in Sync.
This is why it is so important not to edit the sound or video while they are separated.
Let us know how you get on
Mike


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