There is yet another common use of FFMPEG, i.e. adding a new audio track to an existing file which can be a video-only file or a video file already with an audio track. A use case will be adding a second audio track to a file already with video and sound. The following is an example of the command to add audio.dts audio track to an existing mkv file:

ffmpeg -i input.mkv -i audio.dts -map 0 -map 1 -c copy output.mkv