Most of the time, a video or audio file that "won't play" isn't broken — it's just in a format or codec your player doesn't support. The quickest fix is usually to convert it to a widely-supported format like MP4 or MP3. Genuine file corruption is rarer and needs a different approach.
Format vs codec: what's actually wrong
A file like .mp4 or .mkv is a container; inside it, the video and audio are stored with a codec (H.264, HEVC, AV1, and so on). "Won't play" usually means your player supports the container but not the codec inside — or doesn't recognise the container at all. Converting re-wraps and re-encodes the file into something standard that plays everywhere.
The fastest fix: convert to a standard format
For video, convert to MP4 (H.264) — the most universally-supported format there is. For audio, convert to MP3. This resolves the large majority of playback errors in a single step, and in File Transmute it runs in your browser with nothing uploaded.
Common "won't play" situations
- An iPhone .MOV won't play on Windows → MOV to MP4, or read the full fix.
- An .MKV won't play on your phone or TV → MKV to MP4; here's why, and how.
- A voice-memo .M4A won't open → M4A to MP3; see voice memo won't open.
- A video is too large to send (WhatsApp, email) → compress it; see WhatsApp video too large.
- Not sure which format to use? Compare AVI vs MP4 vs MOV vs MKV.
When converting won't fix it
If a file is genuinely corrupted — an interrupted download, a damaged SD card — converting may fail too, because the data itself is incomplete. In that case, re-download or re-export from the original source. Converting fixes format and codec problems, not missing data.
Why does converted audio sometimes sound worse?
Usually because it was converted to too low a bitrate, or a lossy file was re-compressed. It's avoidable — see why does my converted audio sound worse.
Fix it privately in your browser
All of these conversions run on your device — nothing is uploaded. Pick the format you need and convert; for large files, compress the video to fit a device or upload limit.