What happens
- The finished file ends in .webm rather than .mp4, or
- the MP4 plays but has no sound, or
- Add Subtitles to Video says the sound could not be carried into the captioned video and makes no file at all — on purpose, rather than hand you a silent one.
Why it happens
An MP4 file carries its sound as AAC, and the video tools can only write AAC if your browser has an AAC encoder. Most do. Two common setups do not:
- Firefox, on every platform;
- every browser on desktop Linux.
What happens next depends on the tool:
- The Video Compressor and Add Subtitles to Video switch to WebM, which carries its sound in a format these browsers can encode, so your sound is kept.
- The Video Converter lets you pick the format. If the one you pick cannot carry sound in your browser, it says so and names a format that can.
- Add Music to Video cannot do its job without writing sound, so it says so and does not start.
- The other video tools write MP4 only, so they save the video without sound. Most of them say so when you choose the video.
Two other things give a silent result in any browser: a video that never had a sound track, and a very long video whose sound could not be read into memory. Not every tool says which, so if a long video comes out silent in a browser that can write AAC, split it and try the parts.
What to do
- For an MP4 with sound, run the tool in Chrome or Edge on Windows or macOS, or Safari on a Mac.
- Keep the WebM if it suits you. WebM plays in every current browser and in most media players and editors. If you need an MP4 later, convert it with the Video Converter in a browser that can write AAC.
- For a very long video, split it with the Video Splitter and work on the parts.