FFmpeg Formats Documentation
https://ffmpeg.org/ffmpeg-formats.html31.12.2021 · The tee muxer is not useful when using the libavformat API directly because it is then possible to feed the same packets to several muxers directly. Since the tee muxer does not represent any particular output format, ffmpeg cannot auto-select output streams. So all streams intended for output must be specified using -map. See the examples below.