Video Audio Merger
Add audio to a video online for free. Replace the original sound or mix the new track over it — with volume, start offset, fades and loop-to-fit you control exactly how they merge.
Drag & drop your video here
MP4 · MOV · WEBM · AVI · MKV — up to 200 MB / 10 min · processed locally, never uploaded
How to Add Audio to a Video in 3 Steps
A browser audio merger for creators scoring, dubbing or repairing footage.
Upload video and audio
Drop in a video (MP4, MOV, WEBM, AVI, MKV — up to 10 minutes) and the audio track you want (MP3, WAV, M4A, OGG, FLAC). Nothing is uploaded.
Choose how to merge
Replace the original sound or mix over it. Set each track's volume, when the new audio starts, fade in/out, and whether a short track loops.
Download the result
The video stream is copied losslessly — only audio is re-encoded — so the merge finishes fast and your picture keeps its exact quality.
Two ways to merge — plus the knobs between
The merge method is configurable: pick the mode, then shape the timing and levels before you export.
Replace
The original audio is dropped and only your new track remains — the standard fix for swapping in music, a dub or a repaired voice track.
Mix
Keep the original sound and blend the new track over it. Duck the original to 30% for voiceovers or balance both for layered music.
Fit any length
A track shorter than the video can loop until the end or pad with silence; a longer track is trimmed at the video's last frame. Fades in and out are one number each.
Formats: MP4 · MOV · WEBM · AVI · MKV video in — MP3 · WAV · M4A · AAC · OGG · OPUS · FLAC audio in — MP4 / WebM out
When a video audio merger helps
Background music
Drop a licensed track under your footage, set a 2-second fade-out, and export — BGM without opening an editor.
Voiceover ducking
Mix mode keeps the original audio at low volume while your narration rides on top at full level.
Trending sounds
Pair a meme sound or trending audio with your own footage — set the start offset so the punchline lands on the right frame.
Repair damaged audio
Wind noise or a dead mic? Replace the broken track with a clean recording instead of re-shooting.
Multi-language dubs
Export one video per language: replace the original dialogue with each dubbed track from the same upload.
Silent footage
Drone, dashcam, GoPro and screen recordings ship silent — give them a soundtrack in a minute.
Everything you expect from an audio merger — and nothing you don't
- Replace or mix — configurable merge mode
- Per-track volume, start offset, fade in and fade out
- Loop short tracks to cover the whole video
- Lossless video stream copy — the picture is never re-encoded
- Runs 100% in your browser — your files are never uploaded
- No signup, no watermark, no usage limits
Audio in the output: AAC 192 kbps (MP4) or Opus 128 kbps (WebM)
Video Audio Merger FAQ
Is this audio merger free?
Yes. Merging runs entirely in your browser, so there is no server cost to pass on — no account, no watermark and no usage limits.
What is the difference between Replace and Mix?
Replace discards the video's original audio and keeps only your new track — use it for music, dubs or repaired audio. Mix keeps both: the original sound plays (at a volume you set, e.g. ducked to 30%) while the new track plays over it — use it for voiceovers and layered sound.
Will merging reduce my video quality?
No. The video stream is stream-copied untouched — every pixel is identical to your upload. Only the audio is re-encoded (AAC in MP4, Opus in WebM). On the rare occasion a video codec cannot be copied into the output container, the tool falls back to a high-quality re-encode of that file.
What audio formats can I merge in?
MP3, WAV, M4A, AAC, OGG, OPUS and FLAC. Output is MP4 with AAC audio (192 kbps) or WebM with Opus audio (128 kbps).
What happens if my audio is shorter than the video?
Choose: loop the track until the video ends (good for BGM stems), or let the rest play as silence. If the audio is longer than the video, it is trimmed exactly at the video's last frame.
Can I delay the audio so it starts partway into the video?
Yes — set the Start at time and the new track waits that many seconds before it begins. Combine it with fade in for a natural entrance.
What are the length and size limits?
Videos up to 10 minutes and 200 MB, audio files up to 100 MB. Everything is processed in your browser's memory, so very large files depend on your device.
Is my video or audio uploaded to a server?
No. The merger runs on ffmpeg compiled to WebAssembly inside your browser tab. Your files never leave your device — you can even go offline after the page loads.
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Your footage already looks right. Now make it sound right.
Upload a video, drop in a track, choose replace or mix — lossless video, local and free.