Audio out of sync after repairing a video? Causes and fixes

By the FixMP4 engineering team · October 5, 2026 · 6 min read

You repaired a broken recording, the picture looks right, but the voices do not match the lips. Audio sync problems after a repair are rarer than blocky video, but they are frustrating. The good news: they have a small number of causes, and the pattern of the problem tells you which one you have.

Step 1: offset or drift?

Play the repaired file and check sync at the start, the middle and the end.

Constant offset

Most cameras start audio and video at slightly different moments and record an edit list in the index that aligns them. When the index is lost, that alignment has to be inferred. A tool may also need to skip a few frames at the start to begin on a keyframe, which can shift video relative to audio.

Fix: a constant offset of a few hundred milliseconds is easy to correct in any editor by sliding the audio track. If the offset is large, repeat the repair with a reference recorded on the same device in the same mode; FixMP4 takes timing hints from the reference.

Drift

Drift means the video and audio clocks disagree. Typical causes:

• Wrong frame rate: the reference was 30 fps but the broken clip was 29.97 or 25 fps, so video timestamps are computed on the wrong scale. Over ten minutes, 30 vs 29.97 fps adds up to 0.6 seconds.
• Wrong audio sample rate: 44.1 kHz vs 48 kHz produces about 9% drift, which is obvious within seconds.
• Variable frame rate (VFR): phones and screen recorders often vary the frame rate with lighting or load. If the original was VFR, exact timing for each frame lived only in the lost index; a rebuild must assume a constant rate.

Fix: make sure the reference uses exactly the same frame rate and sample rate. See how to choose a reference file. For VFR sources (common with screen recordings and phones in low light) small drift can remain; fixing it in an editor by stretching the audio slightly is the practical answer.

Gaps

A sudden jump in sync means data is missing, typically because part of the card or file was damaged or never written. Video frames after the gap are fine, but the two streams lose alignment at that point. In an editor, cut at the jump and realign the second part.

How FixMP4 handles timing

The repair tool parses every AAC frame to find its exact length, which also gives an exact count of audio frames, each 1024 samples. Audio duration is therefore known precisely from the data itself. Video timing comes from the frame rate, B-frame reordering and keyframe positions. When both streams are correct and the original used a constant frame rate, sync is exact.

Sync issues that are not caused by repair

• The original recording drifted: some cheap cameras and USB microphones drift even in healthy files.
• Bluetooth headphones add latency during playback; check with speakers.
• Editors conforming VFR footage to a constant rate can introduce drift; transcode VFR clips to constant frame rate before editing.

Quick checklist

1. Check whether the problem is offset, drift or a gap.
2. Confirm the reference's frame rate and sample rate in VLC or MediaInfo.
3. Re-run the repair with a better reference (the preview is free).
4. Correct a small constant offset in your editor.

For more on why recordings break in the first place, see why recordings get corrupted.

• audio sync
• AAC
• sample rate
• variable frame rate
• repair