Add Sound to Video Tool

Upload a video and a separate audio file (MP3, WAV, etc.), then either replace the video's original audio entirely with the new track or mix the two together with an adjustable volume for the new track. The video's picture is passed through untouched (no re-encoding, no quality loss) while the new audio is decoded, optionally looped or cut to match the video's length, and muxed into a fresh MP4 or WebM file. Everything runs on-device, nothing is uploaded to a server.

PrivateFreeWorks offline

Reviewed by Vincenzo Manto, Founder & Lead Developer · Last updated October 11, 2026

How it works

  1. 1

    Upload the video you want to add sound to.

  2. 2

    Upload the audio file (MP3, WAV, etc.) you want to add.

  3. 3

    Choose Replace (drop the original audio) or Mix (combine both tracks) and set the new track's volume.

  4. 4

    Pick MP4 or WebM and export. The tool combines the picture with the new audio into a downloadable video.

Lossless video, freshly mixed audio

Instead of decoding and re-encoding the entire video just to swap its audio, this tool copies the video's encoded packets directly into the new file, byte-for-byte identical to the source. Only the audio side goes through decoding, mixing, and encoding, which keeps exports fast and avoids any generational quality loss on the picture.

Real audio mixing, not just stream muxing

When you choose 'Mix', the tool doesn't just place two audio streams side by side (most players would only ever play one). It decodes both the original and new audio into raw PCM samples, sums them sample-by-sample at your chosen volume, and encodes the single resulting track, so both audio sources are genuinely audible together in the final file.

Frequently asked questions

Does this replace the video's audio or combine both?

Your choice. 'Replace audio' drops the video's original audio track entirely and uses only the new file. 'Mix with original audio' keeps both, blending them into a single track at the volume you set for the new one.

Will the exported video's picture quality degrade?

No. The video track is copied packet-for-packet from your original file without decoding or re-encoding, so the picture is bit-for-bit identical to the source. Only the audio is newly encoded.

What happens if my audio file is shorter or longer than the video?

If the audio is shorter than the video, it loops seamlessly to fill the remaining time. If it's longer, it's cut to match the video's exact duration.

Can I control how loud the new audio is?

Yes, a volume slider lets you scale the new track from 0% (silent) to 200% (double gain) before it's mixed in or used as the replacement.

Why does the tool sometimes refuse to export in MP4 or WebM?

Because the video track is copied losslessly rather than re-encoded, the output container must support your source video's exact codec (e.g. an HEVC source can't be packet-copied into a WebM container). If that happens, switch to the other output format.

Is my video or audio file uploaded anywhere?

No. Decoding, mixing, and encoding all happen locally in your browser tab using the WebCodecs and Web Audio APIs. Nothing leaves your device.

Related tools

Screenshot to SaaS Promo Video

Turn one app screenshot into a 3D flyover promo video, like the ones on SaaS landing pages.

Video Trim & Crop Tool

Trim and crop a video in your browser, export as MP4 or WebM with audio kept.

Video Resize Tool

Resize a video to exact pixel dimensions in your browser, export as MP4 or WebM.