Here is another use case for FFMPEG. In some projects, after you mixed and matched video and audio streams from different files, the resultant file may not have the default audio stream as desired. You can use the following command to set the default audio stream while you are mixing the video/audio streams.

ffmpeg -i movie.mkv -i audio.dts -map 0:v:0 -map 1:a:0 -map 0:a:0 -c copy -disposition:a:0 default -disposition:a:1 none movie_combined.mkv