Where the sound went
An MP4 or MOV keeps the audio frames inside the file, between the video frames. What tells a player how to decode them, the AAC audio configuration with its sample rate and channels, sits in the index, the moov atom, written when the recording stops. A recording cut off by a dead battery, a crash or a pulled card never gets it.
Two things can follow:
| What you see | What happened |
|---|---|
| The video won't open at all | The whole index is missing, sound and picture. |
| A repaired or recovered video plays, but silently | The tool that rebuilt it couldn't work out the sound's settings, so it left the audio out. |
| The video plays silently and always did | The recording had no sound: the microphone was off, or the camera records none. Tesla's cameras, for example, record no sound. |
How the checker brings the sound back
AAC sound is cut into frames of 1,024 samples each, and a frame carries enough structure to be checked. The checker reads the audio frames between the pictures, tries the sound settings phones and cameras use, and keeps the one under which the frames decode whole. When enough frames prove the settings, the sound comes back with no second video.
When they don't prove them, the checker plays the picture silently in the preview and offers to bring the sound back with a clip from the same camera, which carries the same audio settings.
What we tested it on
We removed the index from real recordings, repaired them with no second clip, and compared every sound frame with the original:
| Recording | Sound | Result with no clip |
|---|---|---|
| Samsung Galaxy A56, 1080p | AAC | 277 of 277 sound frames identical, and all 352 pictures |
| Our own 1080p test recording | AAC | 705 of 705 sound frames identical, and all 450 pictures |
With a clip from the same camera, a GoPro HERO6 recording came back with all 1,106 sound frames identical, and a GoPro HERO10 recording with all 1,081.
Get the sound back
- Open the broken video in the checker on this page. Your browser reads it from your disk; nothing is uploaded.
- The free check tells you whether it found the footage and the sound.
- Press Repair and listen to the free 5-second preview.
- If the preview is silent and the checker offers a clip, add a video that plays from the same camera with sound, recorded at the same settings.
- Download the full video with a paid plan. If the repaired video doesn't play, you get a refund.
On Android, the Video Repair app does the same on the phone.
What it can't bring back
- Sound that was never recorded: a muted microphone, or a camera without one.
- Sound in formats other than AAC inside MP4 or MOV. AVI files with PCM or ADPCM sound are rebuilt from their own headers.
- Audio that later recordings overwrote.