A music video is built around one audio file, and every frame is positioned relative to it. Keeping that alignment intact through shooting, editing and delivery is a technical discipline.
Playback on set establishes the reference
Performances are shot to playback of the track, so the recorded picture already carries an implicit relationship to a specific version of the audio.
The playback system records its own audio into the camera, giving each take a guide track that an editing system can align against the master.
Without that guide, aligning a performance to the master requires matching lip movement by eye, which is slow and rarely frame-accurate.
Speed changes must be tracked
Shooting at a higher frame rate for slow motion means the playback is often sped up on set so that performance looks correct when slowed down.
The ratio between the shooting rate and the playback rate has to be recorded, because the editor needs it to place the footage correctly.
An error here produces performance that drifts out of sync as a shot progresses, which is far harder to fix than a constant offset.
The master must not change
Once editing begins against a specific audio file, any new version of that file risks shifting the alignment of the entire cut.
Even a remaster that sounds identical can differ by a small amount of leading silence, which offsets everything relative to the picture.
Productions handle this by locking one file as the reference and requiring that any replacement be checked for identical length and start point before substitution.
Sample rate and frame rate do not divide evenly
Audio and video are measured on unrelated clocks, and common frame rates do not correspond to whole numbers of audio samples.
Small rounding differences accumulate over the length of a piece, which is why sync errors often appear only at the end of a video.
Editing systems manage this internally, but the errors reappear when material is moved between systems with different assumptions about the frame rate.
Delivery is where sync is verified
Each destination re-encodes the file, and the audio and video are handled by separate parts of that process.
Checking the delivered file rather than the export is the only way to confirm alignment survived, and the check takes as long as watching the video.
The last few seconds are the most useful place to look, since any accumulated drift is at its largest there and smallest at the start.
A visible sync reference at the head of a delivery file, trimmed before publication, gives anyone downstream an unambiguous way to check alignment without watching the whole piece.