Sound but no picture: the player can't decode H.265
An MP4 can hold H.264 video or the newer H.265, also called HEVC. Many phones, drones and action cameras record H.265 to save space, and a player without an H.265 decoder plays the sound over a black screen or refuses the file.
- On Windows, the built-in players need Microsoft's HEVC Video Extensions from the Microsoft Store. Some PC makers install them; others don't.
- VLC decodes H.265 with no add-on, on Windows, Mac and Android.
- Some older TVs, car screens and media players can't decode H.265. Play the file on a computer, or convert a copy to H.264 with a free converter such as HandBrake. Conversion re-encodes the video, so keep the original.
- To record H.264 from now on, turn off the H.265 option in your camera settings. Samsung names it High efficiency videos.
The copy or download isn't finished
Open an MP4 while it's still copying from a phone or memory card, and the player can fail: many cameras write the index at the end of the file, and the copy hasn't reached it yet. Wait for the copy to finish, then compare its size with the original.
Cloud folders cause the same failure. OneDrive shows a cloud icon on online-only files and downloads them when you open them, which can't happen offline. Right-click the file and choose "Always keep on this device" before you play it. Google Drive and Dropbox have similar online-only modes.
An MP4 that stops at the same point every time, while the player shows the full length, is missing its last part. Files prepared for streaming keep the index at the front, so the player knows the full length and runs out of data at the cut. Download or copy the file again. The missing part never reached this copy, and no tool can rebuild it from here.
Picture but no sound
An MP4 can carry sound in formats other than AAC, such as Dolby Digital (AC-3). Windows 11 version 24H2 dropped the AC-3 codec from new installs, so Windows' own players can play such a file without sound. VLC plays AC-3 without an add-on.
The file may also have no sound track. In VLC, open Tools > Codec Information (on a Mac, Window > Media Information). If it lists a video stream and no audio stream, the file has no sound to play.
A recording that was cut off loses its AAC sound settings along with the index, because the camera keeps them there. If the repair needs them, the tool above asks for a reference clip and brings the sound back with it.
Nothing opens it: the index is missing
An MP4 carries an index, the moov box, that tells players where each frame sits and how long the video runs. Phones and cameras write it when you stop recording. If the battery dies, the app crashes, the card comes out or the power cuts first, the file keeps its size and loses the index. Players refuse it with messages such as error 0xc00d36c4 on Windows or "Can't play video" on Android, and its properties show no length.
VLC's repair option covers AVI files and can't rebuild an MP4 index. The tool above rebuilds the index from the footage still in your file.
Repair the MP4 in your browser
- Choose the MP4 in the tool above. It reads the file from your own disk, and nothing uploads.
- The free check reports whether it found footage it can rebuild.
- If the tool asks for a reference clip, add any healthy video from the same camera at the same settings. The reference also brings back missing sound.
- Watch the free 5-second preview, at reduced quality with a watermark.
- Save the full video with a plan: one video, three months or a year, cancel anytime. If the repaired video doesn't play, you get a refund.
The repaired file keeps the camera's video and sound data byte for byte, with a new index written around it, and the tool re-encodes nothing. It can't undelete files or rebuild footage that was overwritten or never written, such as a file that is empty or all zeros. It doesn't open AVI, MKV, TS or WebM files yet, and it can't decrypt videos hidden by vault apps.