BYTETOOLS

Add Audio to Video

Add music or a voice-over to a video online, free. Replace the original soundtrack or mix the new audio into it — in your browser, nothing uploaded.

Drop your video hereProcessed locally — never uploaded.
Drop the audio track hereMP3, WAV, M4A, OGG and more.

What is the Add Audio to Video?

The ByteTools Add Audio to Video tool puts a new soundtrack on any clip. Drop in a video and an audio file (MP3, WAV, M4A, OGG and more), then choose Replace to make the new audio the only soundtrack, or Mix to blend it with the video's existing sound — ideal for background music under a voice-over.

  • Replace mode swaps the soundtrack without re-encoding the picture
  • Mix mode blends music under the original audio
  • Accepts MP3, WAV, M4A, OGG and other common audio formats
  • Output ends at the shorter input — no trailing silence
  • 100% local processing — files never uploaded
  • Standard MP4 (AAC audio) output

How to use the Add Audio to Video

  1. 1

    Drop your video into the left box and the audio file into the right box.

  2. 2

    Choose Replace to swap the soundtrack, or Mix to blend the new audio with the original.

  3. 3

    Tick “Re-encode video” if your source is a WebM or AVI file.

  4. 4

    Click Add audio to video, preview the result and download the MP4.

About the Add Audio to Video

The ByteTools Add Audio to Video tool puts a new soundtrack on any clip. Drop in a video and an audio file (MP3, WAV, M4A, OGG and more), then choose Replace to make the new audio the only soundtrack, or Mix to blend it with the video's existing sound — ideal for background music under a voice-over.

In Replace mode the video stream is copied without re-encoding whenever possible, so the operation is fast and the picture quality is untouched. The output stops at the shorter of the two inputs, so a 30-second clip with a 3-minute song simply ends after 30 seconds.

Everything runs 100% locally in your browser with a WebAssembly build of ffmpeg — neither your video nor your audio file is uploaded anywhere, no account is needed and no watermark is added. The result is a standard MP4 with AAC sound that plays in every browser, phone gallery and editing app.

Frequently asked questions

How do I put music over a video?

Drop the video and the music file onto this tool and click Add audio to video. Replace mode makes the music the only sound; Mix mode plays it together with the video's original audio, which is what you want for background music under speech.

Will adding audio reduce my video quality?

In Replace mode the video stream is copied bit-for-bit, so the picture is completely untouched — only the audio changes. Mix mode also copies the video by default. Only the optional re-encode setting (needed for WebM/AVI sources) touches the picture at all.

What happens if the audio is longer than the video?

The output stops when the shorter input ends, so the song is cut off when the video finishes. If you want a specific section of the song, trim it first with the Trim Audio tool, then add it here.

Why does Mix mode fail on my clip?

Mix mode blends the new audio with the video's existing soundtrack, so it needs the video to have one. If your clip is silent, use Replace mode instead — the result is the same, since there is nothing to mix with.

Are my files uploaded to a server?

No. The muxing and mixing run inside your browser with WebAssembly. Both files stay on your device, which means no upload wait, no size limits and full privacy.

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