Does this reduce the video quality?

No. The video stream is copied without being decoded or re-encoded, so every frame in the output is bit-for-bit the frame from your original. Only the audio is written fresh, and only when you leave the audio option on AAC.

What happens if the audio is longer than the video?

That is what the length option decides. 'Stop at whichever track ends first' cuts the longer one so the file ends when the shorter track does. 'Run until both tracks have ended' keeps everything — if the audio is longer, the file continues with sound after the last frame.

Why can I not use a WebM video here?

Because the video stream is copied rather than converted, and MP4 cannot reliably carry WebM's VP8 or VP9 video. Convert the WebM to MP4 first, then add the audio.

Can I keep the original audio and add music underneath?

Not here. This tool replaces the audio track. Mixing two tracks needs level balancing to be useful, and a tool that silently mixed at equal volume would produce a worse result than it promised.

Are my files uploaded?

No. The first time you use an audio/video tool, your browser downloads the open-source FFmpeg engine — a 32 MB file, roughly 10 MB once your connection compresses it — as a static file, and that is the only network traffic. Your media files are then processed entirely on your device and never reach a Filesvo server.

Does this work on phones?

Yes on recent devices, with limits: the engine is a 32 MB file (roughly 10 MB compressed in transit), and phones have less memory available to the browser, so large videos can fail on mobile. If a file fails on your phone, try a smaller file or a desktop browser. The 100 MB per-file limit applies everywhere.

Add Audio to a Video Online

Drop in a video and an audio file and get one MP4 back. The picture is copied across stream-for-stream — no re-encoding, no generation loss, and far faster than exporting from a video editor. Everything runs in your browser.

1. The video

or drag and drop a .mp4 / .m4v / .mov file here — up to 100 MB

2. The audio to add

or drag and drop a .mp3 / .wav / .m4a / .ogg / .flac file here — up to 100 MB

This tool runs the open-source FFmpeg engine in your browser. First use downloads it once (a 32 MB file, roughly 10 MB compressed in transit); after that it loads from your browser cache. Your files are never uploaded. The engine is ffmpeg.wasm, distributed unmodified under the GPL-2.0-or-later.

Processed on your device — files are never uploaded to Filesvo.

Add audio to a video with FFmpeg — The command everyone copies keeps the wrong audio. Here is what it does, measured, and the fix.

How to use Filesvo

Select or drop the video — an MP4, M4V or MOV.
Select or drop the audio to add — MP3, M4A, WAV, OGG or FLAC.
Choose how to handle the audio track and what to do if the lengths differ, then click Add Audio to Video.

Why nothing is re-encoded

A video file is a container holding separate streams. Swapping the audio stream does not require touching the video stream at all, so this tool copies the picture across byte-for-byte and only writes a new audio track beside it. That is why it finishes in seconds on a file a video editor would spend minutes re-exporting — and why the result is visually identical to your original rather than a second-generation copy.

The audio is re-encoded to AAC at 192 kbps by default, because AAC is the codec an MP4 can carry from any source you might drop in. If your audio is already AAC (.m4a) or MP3, switch the audio track option to copy and nothing at all is re-encoded.

Whatever audio the video already had is replaced, not mixed. Mixing two tracks together is a different job and this tool does not pretend to do it.

The FFmpeg command this runs

This tool is FFmpeg itself, compiled to WebAssembly and running in your browser. Below is the command it builds with the options set as they arrive — generated from the same code that performs the conversion, not written out by hand, so it cannot drift out of date. Paste it into a terminal with FFmpeg installed and it does the same job there.

ffmpeg -y -i video.mp4 -i audio.m4a -map 0:v:0 -map 1:a:0 -c:v copy -c:a aac -b:a 192k -shortest -movflags +faststart output.mp4

The two -map flags are the ones that matter: without them FFmpeg picks the audio stream itself and usually keeps the video's original track.

What each option on the tool changes in that command:

Audio handling

Re-encode to AAC — -c:a aac -b:a 192k

Copy the stream — .m4a and .mp3 only — -c:a copy

If the lengths differ

End with the shorter input — -shortest

Run to the longer input — adds no flags

The input and output names are placeholders. Your own filenames are used inside the browser, and neither file leaves your device.

Private by design

Your files are processed directly in your browser using the open-source FFmpeg engine compiled to WebAssembly and are not uploaded to Filesvo. Nothing is stored on our servers, because nothing ever reaches them.

The engine is a static file your browser downloads once and then caches — the only network request this tool makes. It is used exactly as its authors published it; see the engines and their licences.

Frequently asked questions

Does this reduce the video quality?: No. The video stream is copied without being decoded or re-encoded, so every frame in the output is bit-for-bit the frame from your original. Only the audio is written fresh, and only when you leave the audio option on AAC.
What happens if the audio is longer than the video?: That is what the length option decides. 'Stop at whichever track ends first' cuts the longer one so the file ends when the shorter track does. 'Run until both tracks have ended' keeps everything — if the audio is longer, the file continues with sound after the last frame.
Why can I not use a WebM video here?: Because the video stream is copied rather than converted, and MP4 cannot reliably carry WebM's VP8 or VP9 video. Convert the WebM to MP4 first, then add the audio.
Can I keep the original audio and add music underneath?: Not here. This tool replaces the audio track. Mixing two tracks needs level balancing to be useful, and a tool that silently mixed at equal volume would produce a worse result than it promised.
Are my files uploaded?: No. The first time you use an audio/video tool, your browser downloads the open-source FFmpeg engine — a 32 MB file, roughly 10 MB once your connection compresses it — as a static file, and that is the only network traffic. Your media files are then processed entirely on your device and never reach a Filesvo server.
Does this work on phones?: Yes on recent devices, with limits: the engine is a 32 MB file (roughly 10 MB compressed in transit), and phones have less memory available to the browser, so large videos can fail on mobile. If a file fails on your phone, try a smaller file or a desktop browser. The 100 MB per-file limit applies everywhere.

MP4 to MP3

Extract the audio track from an MP4 video as an MP3 file — entirely in your browser with the FFmpeg engine.

Video Trimmer

Cut a clip out of a video with a live preview — frame-exact re-encode or instant lossless keyframe copy.

Compress Video

Shrink MP4, MOV and WebM videos with a quality you choose, plus an optional 720p cap — with honest before/after sizes.