Add another audio track to a video file with FFmpeg. This will give you the option to change the audio track in a video player whilst keeping the video flowing.
A concept that is most seen for movie files and different language audio.
ffmpeg -i input.mp4 -i audio.mp3 -map 0 -map 1 -c copy output.mp4
-map flag means that you can specify which streams to keep.
-map 0 is input.mp4 whilst
-map 1 is audio.mp3
The streams of both inputs are copied together as
-c copy means no re-encoding is done.