Video plays but there's no sound: fixing an unsupported audio codec

The picture is fine, so the video stream is fine. The silence is an audio track your player cannot decode — usually AC-3 (Dolby Digital), E-AC-3, DTS or FLAC.

The repair is to re-encode only the audio to AAC and copy the video across untouched: seconds of work, and every pixel stays bit-for-bit identical. The browser-based MKV to MP4 converter does this on your own machine, without uploading the file.

First, rule out the boring causes

Thirty seconds of checking saves you a conversion. The problem is not the codec if the file plays with sound in a different player on the same computer, if the player's own volume is at zero or the browser tab is muted, if Windows is routing audio to a monitor or a headset that is off, or if the file has several audio tracks and the player picked a silent or commentary one.

If the file is silent on every player, on every device, and on the web, the audio codec is the cause. That is what this page fixes.

Which audio codecs go silent, and where

Video codecs get all the attention, but audio has exactly the same licensing problem. AC-3, E-AC-3 and DTS are patent-encumbered formats that a manufacturer has to pay to include. Software that ships without those licences simply has no decoder, so it plays the video and outputs nothing.

AAC is the opposite case. It is the codec MP4 was designed around, and it is decoded by every browser, phone, smart TV, editor and car head unit worth naming. That is why the fix is always "make it AAC", whatever the source was.

Why "install a codec pack" is the wrong fix

Every search result for this problem tells you to install AC3Filter, a codec pack, or VLC. Those do work, and VLC in particular is a good player precisely because it carries its own decoders for everything. But look at what has actually been fixed: one application, on one computer.

The file has not changed. Email it to someone and it is silent again. Put it on a USB stick for the TV and it is silent again. Drop it into an editing timeline and you get a video track with no waveform. The fault travels with the file, so the fix has to be applied to the file — once, permanently.

The fix: re-encode the audio, copy the video

This is the part nearly every converter gets wrong. Given a file with a bad audio track, most tools re-encode the whole thing — picture included — because that is the only mode they have. The picture is already fine, and re-encoding it is far slower and throws away quality for nothing. What you want is FFmpeg's -c:v copy -c:a aac: keep the video stream byte for byte and rebuild only the audio.

1. Open the converter and drop your file in. It reads the file locally and reports the video codec, the audio codec and the channel layout, so you can confirm the audio really is AC-3, DTS or FLAC before you change anything.
2. Choose MP4 as the output.
3. Open the advanced options and set Audio to Re-encode. Leave resolution and frame rate on Original — changing either of those forces the video to be re-encoded too, which is exactly what you are trying to avoid.
4. Check the summary before you convert. It should read Video copied, audio re-encoded. That line is your guarantee the picture comes through untouched.
5. Convert. Because only the audio is rebuilt, this finishes quickly even on a long film.

What changes and what doesn't

The picture does not change at all. A stream copy is not "high quality" — it is the same data. No generation loss, no softening, no banding.

The audio is genuinely re-encoded, and that is lossy. In practice you are unlikely to hear the difference on a Dolby Digital soundtrack, because AC-3 was itself lossy. But be precise about it: this step is not lossless, and no setting makes it lossless.

The new track is AAC at 128 kb/s. That is what this repair writes, and it is fixed, because the quality chips are not an audio control: choosing Low, Medium or High switches the whole job to a full re-encode and the picture you were copying gets rebuilt with it. A higher audio bitrate is only available at that price, and on a 5.1 Dolby Digital source — itself lossy to begin with — it is rarely worth paying.

The file size barely moves. Audio is a small fraction of a video file, so the output lands close to the input. Making it smaller is a separate job, and that one does mean re-encoding the picture.

Extra tracks are dropped. Only the first video and first audio stream survive; subtitles and additional audio tracks are not carried over, and the tool tells you how many it is discarding. If the track you want to hear is the second one, this repair will not help.

When the silence is not a codec problem

There is no audio stream at all. Screen recorders are the usual culprit: system audio capture is off by default in many of them, so you get a perfect video with nothing recorded alongside it. Drop the file into the converter and the file details will list the audio track as None. Nothing can add sound that was never captured.

The file is DRM-protected. Purchased or downloaded-from-a-service files can carry encryption that no converter will open, ours included.

The problem is the app, not the audio. Some applications reject a file for reasons unrelated to codecs and fall back to a strange half-working state. DaVinci Resolve, for example, has its own particular complaint about certain MP4s, covered in the guide on Resolve importing an MP4 as audio only, and Premiere has its own with Matroska in why Premiere Pro won't import MKV. If a file is silent in one editor but plays with sound in every player you own, start with those.

If your video lost its sound after converting somewhere else

This is common enough to be worth its own note. A converter that produced a silent output usually did one of three things: it dropped the audio track because it could not decode the source codec, it wrote the audio into a container that cannot legally hold it, or it kept only a track your target device does not understand.

The diagnosis is the same either way — check what is actually inside both files. The guide on checking what codec your video really uses walks through reading the container, the video codec and the audio codec from a file without installing anything. Compare the original with the broken output and the missing step is usually obvious in seconds.

If the original still has its audio, come back to the audio-only repair above rather than running the file through a second full conversion. Stacking re-encodes is how a good source ends up looking mushy. The same reasoning is spelled out at length in converting MKV to MP4 without losing quality.

Fixing audio for a specific destination

Some destinations are strict about audio in ways that are easy to mistake for a broken file. Chat and social apps expect AAC and will either refuse the file or play it mute — see converting a video so WhatsApp will send and play it. Televisions reading from a USB stick behave the same way, covered in getting an MKV to play on a smart TV: a set that shows a DTS film in silence will often play it happily once the track is AAC.

On a phone it is different. The iPhone app and the Android app use the device's hardware encoder, which makes them far quicker than any browser on long or 4K footage — but they always re-encode and have no audio-only mode. To repair sound while keeping the picture untouched, use the browser tool.