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Is it possible to embed a thumbnail in a .mkv file?
I was also going down this rabbit hole and found that there's an option -attach filename that's more specific to mkv files. It's at the end of the "Main Options" section of the manual: https:...
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Accepted
Mixing audio with HDMI video to replace the sound from the camera
It seems that what you are looking for is an HDMI audio embedder.
It seems that the Tascam DR-701D offers this functionality as described in Outputting this unit's audio as the HDMI output audio
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