Replace the audio track in a video
You have a video with an audio track (call it
video.mp4). You have an audio file (e.g.
audio.mp3) that you want to put on the video, replacing the original audio of the video file.
With Audacity, you can open both
audio.mp3 and you can synchronize the
mp3 to match the original audio. Important points: speech/music should start at the same time, and its length should be the same or a little bit less than the original audio.
Once you have a good
audio.mp3 file, do the replacement with the following command:
ffmpeg -i audio.mp3 -i video.mp4 -c copy final_video.mp4
Before the previous solution, I used Avidemux, but it tends to crash sometimes :) When I have such videos where Avidemux crashes, I use ffmpeg from the command-line.