ffmpeg copy video or audio only:
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This is where you only want to convert and transfer the video or audio only.
For example you may want to convert the video only, so audio can be done separately with nero aac on soundKonverter instead, then merged the separate new video and audio on MKVtoolnix. Or sound only if you want to have an audio only copy of the file, with out any conversion.

For Video only:
You will use the "-map 0:v" flag to pick up the video file only to convert and pass to new file.
ffmpeg -i old.WMV -map 0:v -c:v copy  new.MKV

For Audio only:
The same as before, but instead of v (for video) use a (audio):
ffmpeg -i old.WMV -map 0:a -c:a copy  new.MKV

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Source: https://stackoverflow.com/questions/12368151/adding-silent-audio-in-ffmpeg
Date: September 7th 2021.