On a music video set the song being played back is frequently not at its correct speed. The mismatch is deliberate and it is what allows slow-motion and accelerated shots to remain in sync.
Off-speed filming changes the duration of everything
A camera recording more frames per second than playback uses produces slow motion, because the same action is spread across more frames of screen time.
A performance filmed that way, with the track at normal speed, would play back slowly on screen and no longer match the song at all.
The correction is to speed the playback up on set by the same factor, so that when the footage is slowed to normal rate the performance lands correctly.
The performer works against an altered track
An artist miming to a track running noticeably fast has to deliver the words at that speed, which is physically difficult and sounds nothing like the song.
Rehearsal is essential, because the timing has to be exact for the lips to match once the footage is returned to normal rate.
Small errors are magnified by the speed change, so a slight lag at high speed becomes an obvious mismatch after the shot is slowed down.
Slowed playback produces the opposite effect
Shooting at a reduced frame rate and playing the track slowly gives an accelerated final image, with movement that appears faster than it could naturally be.
The technique produces a distinctive quality of motion, because the performer moves at an unusually deliberate pace and the speed-up compresses it into something unnatural.
Combining both approaches within a single video is common, and it is why the same performance can feel weightless in one shot and urgent in the next.
Sync is verified on set rather than assumed
The alignment cannot be judged by eye at the recording speed, since the picture and the audio are both wrong at that moment.
Productions therefore check playback at final speed during the shoot, rather than discovering a sync failure in the edit when the performer has gone.
Timecode or a clear sync reference on each take makes this practical, and it is one of the reasons music video shoots are more technically procedural than they appear.
The edit inherits the constraint
Every shot arrives with a speed change already baked into how it must be interpreted, and applying the wrong factor puts the performance out of sync immediately.
Careful labelling of the recording rate per take is therefore essential, because the correct treatment cannot be inferred reliably from the footage itself.
Where records are poor, editors resort to matching the lips manually against the track, which is slow work and rarely produces a result as clean as the original intent.