Concretely, the audio track is dropped from the container and the video stream is copied through untouched, so the operation is instant and the picture is bit-identical to the source rather than re-encoded. The result is the same picture with no audio track at all, rather than an audio track turned down to zero.
Is the video re-encoded when I mute it?
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No. Removing a track is a container operation, so the video stream is copied through bit-identically and the job finishes in seconds regardless of how long the clip is.
Does the container format matter?
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It does. M4V is Apple's MP4 variant, sometimes with FairPlay DRM attached, otherwise structurally an MP4. That governs how the tracks are stored and therefore how cleanly one can be dropped.
Anything specific to M4V worth knowing?
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Yes — M4V is structurally an MP4, but a FairPlay-protected one cannot be processed at all — only unprotected files go through. Relevant when the source has more than one audio track.
Can I remove just one audio track and keep another?
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Muting removes the audio entirely. If a file carries several language tracks and you want to keep one, that is a track selection job rather than a mute, and it needs the conversion tools instead.
Should I mute or just silence the audio?
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Mute. A silent track still occupies bitrate and still has to be decoded by the player; dropping it makes the file smaller and removes any chance of it being unmuted downstream.
