DEMUX and MUX

I could do this job using FFmpeg.

With a video named "video.mp4".
And a new (longer) soundtrack named "soundtrack.m4a".

Remove the old soundtrack from the video.
Like this:-

Code:

ffmpeg -i video.mp4 -vcodec copy -an silent_video.mp4

Then add new soundtrack using "-shortest".
Like this:-

Code:

ffmpeg -i silent_video.mp4 -i soundtrack.m4a -shortest -vcodec copy -acodec copy new_video.mp4