Short answer: work down this list in order. A video with no sound in it, an image picked by mistake, and a recording too long for the browser's memory are the three that catch most people, and all three look identical from the outside - the file just refuses to open.
Everything here applies to browser-based audio tools generally. The specifics are from Candid Tools, where the audio never leaves your device, which changes which limits you run into.
1. Check the video actually has sound
This is the most common cause and the least obvious, because nothing about the file looks wrong.
Screen recordings are the usual culprit. Most screen recorders capture system audio only if you turn it on, so a clip that plays fine - with silence - has no audio track at all. Some editing apps also export video-only when a track is muted at the moment of export.
How to check: open the file in any video player and look for a volume slider or audio track. No track means there is nothing to extract, split or transcribe, and no tool can add one back.
Fix: re-record or re-export with audio capture switched on. If the sound only exists in the original project file, export from there rather than from the finished video.
2. Make sure it is actually an audio or video file
Phone galleries make this easy to get wrong. You open the picker, see a grid of thumbnails, and tap the wrong one - a screenshot or a photo instead of the recording.
It is also worth knowing that the file picker's format filter is only a hint. Drag-and-drop ignores it, and Android's picker frequently ignores it too, so a .jpg can reach a tool that only reads audio.
Fix: check the file extension before you upload. If it ends in .jpg, .png, .heic or .pdf, it is not the file you want.
3. Phone videos: .mp4 from Android, .mov from iPhone
Phone video is the format most likely to be refused, for two separate reasons.
On Android, the camera writes .mp4, but the audio inside can use a codec the browser's decoder does not accept - particularly on older Android versions or manufacturer camera apps that deviate from the standard.
On iPhone, the camera writes .mov. Safari's built-in audio decoder refuses iPhone .mov outright, even though the file is perfectly valid and plays in the Photos app.
Good tools work around both by streaming the file and decoding only the audio track, rather than handing the whole thing to the browser's own decoder. When that still fails, the codec is genuinely unsupported.
Fix: convert it first. MP4 to MP3 and MOV to MP3 both run in the browser and produce a file every audio tool can read. Converting also shrinks the file substantially, which helps with the next problem.
4. The recording is too long for the browser
A long recording is not rejected for being the wrong format. It is rejected because the browser ran out of memory holding it.
Audio decoded for processing is uncompressed. An hour of stereo at CD quality is roughly 1.2 GB in memory - before the tool does anything with it. A phone will give up well before that.
Rough guidance for in-browser tools:
- Up to about 20 minutes - fine on most devices, including phones
- 20 to 60 minutes - usually fine on a computer, unreliable on a phone
- Over an hour - expect failures unless you split it first
Fix: trim to the part you need with Trim Audio, or split it into shorter pieces and process them one at a time. Converting a large video to MP3 first also helps, because the audio arrives without the video alongside it.
5. An unusual or older format
Formats like .wma, .flv, .amr, .aiff and camcorder formats like .mts are not supported by browser audio decoders at all. Some transcription tools bundle a converter and handle them anyway; most audio tools do not.
Fix: export as WAV or MP3 from whatever app produced the file. That one step resolves more "could not be decoded" errors than anything else on this page, because it replaces an unusual container with one every decoder understands.
What works without any conversion
| Format | Extensions | Notes |
|---|---|---|
| Lossless | .wav, .flac |
Always works. Large, so mind the length limits above |
| Compressed audio | .mp3, .m4a, .aac, .ogg, .opus |
The safest everyday choice |
| Video containers | .mp4, .m4v, .mov, .webm, .mkv |
The audio track is read directly - as long as there is one |
If your file is not in that table, converting to MP3 or WAV first is the reliable path.
Why a browser tool hits limits a website does not
Most online audio tools upload your file to a server, process it there, and send a result back. The server has far more memory than your phone and a full set of codecs, so it rarely refuses anything - and it has a complete copy of your recording.
A browser-based tool does the work on your own device, so nothing is uploaded. The trade is that the limits become your device's limits: its memory, and the codecs its browser happens to ship. That is why a two-hour recording can fail locally and succeed on a site that uploads it.
For a voice memo, a song or a meeting recording, that trade is usually worth making. For a two-hour file, convert and trim first, and it works fine.
Still stuck?
If the file is a reasonable length, has sound, and is in a supported format but still will not open, it may be damaged partway through - common with recordings interrupted by a crash or a full disk. Try playing it all the way to the end in a media player. If playback stops early, the file is truncated, and re-exporting from the original source is the only fix.