Compressed video won’t play? Work through these checks

Start with the actual downloaded file

1. Read the result message. Note whether the target was met and whether the tool reported discarded sound or a fallback codec.
2. Play the page preview, then download the result and open that saved file locally. A working preview does not verify that the download or later transfer completed.
3. Check that the saved file has a plausible nonzero size and has finished downloading. A tiny file is a clue, not proof of one particular cause.
4. Test the same saved file in another trusted player or on the intended device. Record which combinations succeed.
5. If the saved file is incomplete, download the existing result again while the page remains open. If conversion failed, return to the original for a new attempt.

Playback in one player demonstrates that it can read the tested content; it does not certify that the file is undamaged throughout or compatible with every destination. Do not install unknown codec packs to work around a file from an untrusted source.

An MP4 name is not a compatibility guarantee

MP4 is the container. Inside it, the video and audio have their own codec and configuration requirements. A player can accept MP4 in general but reject the particular video profile, dimensions, audio or other properties. Renaming an extension does not convert those streams.

This implementation tries H.264 video first, followed by VP9, AV1 and HEVC when supported. Audio encoding tries AAC, then Opus. Available decoding and encoding depend on the browser, operating system and device; being able to play a codec does not imply being able to create it. See Mediabunny’s support documentation.

Read the reported output codecs and the destination’s specification. H.264 with AAC is a useful compatibility starting point, not a promise for every TV, browser or editor. An updated browser on another device may offer a different encoder, but no browser name guarantees a particular output. If a strict delivery format is required, use a supported export workflow that lets you verify it.

If the picture plays but sound is missing

• Check player mute, volume and output-device settings, then compare with the original.
• Confirm that the source contains the audio you expect and that Sound was set to Keep sound.
• Inspect the result for a missing-audio warning or codec detail. Keep sound is a request, not a guarantee that the browser can decode and encode the track.
• Try the saved file in another supported player. If sound works there, investigate the destination’s audio support.
• If necessary, retry from the original in another supported environment and check the actual result.

The converter selects primary tracks. Do not assume that alternate language tracks, subtitles or supplementary audio will survive. If sound is essential, verify it before sharing rather than relying only on the success status.

Use the symptom to choose the next comparison

Keep the tab visible during conversion and avoid closing or reloading it before saving the result. Large outputs are buffered in memory, so a file within the input limit may still fail on a constrained device. Processing time estimates are approximate; another device or a lower resolution may help, without guaranteeing success.

Record enough detail for useful support

Record the browser and OS version, source format, dimensions, duration, chosen settings, displayed output codecs, byte count and exact error. Describe whether the source, preview and downloaded file each play. Share only information you are comfortable disclosing; do not publish private footage to demonstrate a fault.

If the only copy is damaged, preserve it before attempting repair. Recoverability depends on the missing or damaged data; there is no universal repair promise. Where an intact source exists, a fresh export is usually the clearest comparison. For quality problems rather than playback failure, use the quality checklist.