Recently I had a corrupt audio-stream in one of my videos. So, there was the need to extract the video-stream from a mp4 container and combine it with a fixed audio stream. Here are the ffmpeg commands I used for that - maybe someone needs them too.

Extract audio only

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ffmpeg -i inputfile.m4v -vn -acodec copy outputfile.aac

Extract video only

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ffmpeg -i inputfile.m4v -an -vcodec copy outputfile.h246

Merge video and audio

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ffmpeg -i audio.aac -i video.h246 -acodec copy -vcodec copy output.m4v

Hope it helps,
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