ffmpeg -i inputfile
returns:
Stream #0:0: Video: hevc ..... Stream #0:1(eng): Audio: aac .... Stream #0:2(eng): Subtitle: ... Stream #0:3(fre): Audio: aac .... Stream #0:4(fre): Subtitle: aac ....
ffmpeg -i inputfile.mkv -map 0:3 french-audio.mp3
NOTE: This extracts the 3rd stream.