ffmpeg#-c copyすべてのストリームをコピー

ffmpeg -i test.mp4 -c copy output.mkv

-c copy WILL copy all the streams.すべてのストリームをコピーすることを示します.

-c[:stream_specifier] codec (input/output,per-stream)
-codec[:stream_specifier] codec (input/output,per-stream)
Select an encoder (when used before an output file) 
or a decoder (when used before an input file) for one or more streams. 
codec is the name of a decoder/encoder or a special value copy (output only) to indicate that the stream is not to be re-encoded.

For example

ffmpeg -i INPUT -map 0 -c:v libx264 -c:a copy OUTPUT

encodes all video streams with libx264 and copies all audio streams.
For each stream, the last matching c option is applied, so

ffmpeg -i INPUT -map 0 -c copy -c:v:1 libx264 -c:a:137 libvorbis OUTPUT

will copy all the streams except the second video, which will be encoded with libx264, and the 138th audio, which will be encoded with libvorbis.
References:
https://ffmpeg.org/ffmpeg-all.html