The WEBA file (.weba)

A .weba file is a WebM file that contains only audio. WebM is the royalty-free media format of the web, and when there's no picture, its audio track is almost always Opus, sometimes Vorbis. The a at the end just marks it as audio, the way .oga marks Ogg audio.

The WebM project itself uses .webm for every file, with or without video. .weba is a convention that comes from media type tables: MDN's list of common types, for example, pairs .weba with audio/webm and .webm with video/webm. Software that picks a file name from the media type, and some converters and recording apps, therefore save audio-only WebM as .weba. In our test, Chrome itself named a downloaded audio/webm file .webm, so you'll find the same audio under both names.

Where WEBA files usually come from:

• Recordings made in a browser. Voice recorders, podcast tools and dictation apps that run in a web page use the MediaRecorder API. Chrome and Edge record audio as audio/webm; codecs=opus; Firefox can record WebM or Ogg; Safari can record WebM since version 18.4.
• Audio extracted from WebM videos without re-encoding: the Opus track is copied into a new, audio-only WebM.
• Converters that offer WebM as an output format for audio.

Everything in WebM is an EBML element: an ID, a size and the content, with variable-length numbers so that small values take one byte. A complete audio-only WebM has the same top-level elements as a video: Info (timing and duration), Tracks (one audio track), Clusters with the compressed audio, and Cues for seeking. The WebM explainer goes through a full video file element by element.

Opus audio is the same whether it's in WebM or in Ogg; only the wrapping differs. That's why moving it between .weba and .opus with FFmpeg's -c copy is lossless and instant.

MediaRecorder hands audio to the page in slices while recording. The browser can't go back and change bytes it has already handed over, so it writes the Segment and Clusters with unknown sizes and leaves out Duration and Cues, as in the anatomy above. The result is a valid WebM file built for streaming, but players show no length or a wrong one, seeking is slow or impossible, and some editors refuse it.

In our test, recent Chrome added Duration, sizes and Cues when the whole recording was collected in one piece at the end (MediaRecorder without a timeslice). With 100 ms slices, which recording apps need for long sessions and uploads in progress, the file had none of them.

The fix is a remux: copying the audio into a new file, which writes the missing elements without touching the sound.

# rewrite a browser recording with a duration and a seek index
ffmpeg -i recording.weba -c copy -f webm fixed.weba

Renaming a .weba file to .webm is harmless and sometimes helps software that doesn't know the .weba extension. Renaming it to .opus or .ogg isn't enough: those are a different container, so use ffmpeg -i in.weba -c copy out.opus instead.

We checked playback of both Opus and Vorbis .weba files served as audio/webm in Chrome 141 and Firefox 142. For a page that must also work in apps that ignore WebM, offer an M4A (AAC) or MP3 copy:

<audio controls>
  <source src="note.weba" type='audio/webm; codecs="opus"'>
  <source src="note.m4a" type="audio/mp4">
</audio>

• No duration, or seeking doesn't work: it's a browser recording without Duration and Cues. Remux it with ffmpeg -i in.weba -c copy -f webm out.weba.
• FFmpeg says "Unable to choose an output format": FFmpeg doesn't recognise .weba as an output name (we saw this with FFmpeg 6.1). Add -f webm before the output file.
• Won't open in Apple Music, QuickTime, Windows apps or a chat app: they don't read WebM. Convert to M4A or MP3, or play it in a browser or VLC.
• An upload form rejects it: the form may only accept common extensions, or check for video/webm. Rename it to .webm, or convert it.
• Sounds fine but the file is unexpectedly large: in our test, Chrome recorded at about 128 kbps stereo, plenty for music but far more than a voice needs. Re-encode speech at 24 to 32 kbps mono Opus.

• Play it: drag it into Chrome, Edge, Firefox or Safari, or open it with VLC or mpv on Windows, macOS or Linux.
• Edit it: Audacity imports WebM audio. Export to WAV for editing, then to the format you need.
• Inspect it: ffprobe file.weba shows the codec, channels and whether a duration is known; MKVToolNix's mkvinfo lists every element.

# to MP3 (re-encodes)
ffmpeg -i in.weba -c:a libmp3lame -q:a 2 out.mp3

# to M4A with AAC (re-encodes)
ffmpeg -i in.weba -c:a aac -b:a 160k out.m4a

# to an Ogg Opus file, without re-encoding
ffmpeg -i in.weba -c:a copy out.opus

# to WAV for editing
ffmpeg -i in.weba out.wav

Converting to MP3 or M4A is lossy to lossy, so use a good bit rate. Copying into .opus keeps the original sound exactly.