[FFmpeg-user] Replacing First 90 Seconds of a Video with Silence or with Another Audio Clip

[Sam Logan]

I have a video AVI file, "Video.avi", consisting of a video and an audio
track, and a separate audio file, "Audio.mp3". I want to do the following
two things:

(1)

First, I want to produce a version of the video, "OutputSilenced.avi",
which would be identical to the original video except that the first 90
seconds of the video's audio are replaced with silence (to clarify, I want
to REPLACE, as in overwrite, the first 90 seconds, NOT insert 90 seconds of
silence to push the audio back 90 seconds).

(2)

Second, I want to produce a version of the video, "OutputReplaced.avi",
which would be identical to the original video except that the first 90
seconds of the video's audio are replaced with the first 90 seconds of
"Audio.mp3" (again, to be clear, I want to REPLACE, as in overwrite, the
first 90 seconds, NOT insert 90 seconds of audio to push the original audio
back 90 seconds).

How commands would I use to do this with FFmpeg? Ideally, I would like to
just remux without doing any re-encoding. Thanks in advance.