Merge Audio and Video Online to MP4 – Fast Client-Side Muxer

Combine any video with a separate audio track into a single MP4 file instantly, right inside your browser — completely client-side with zero backend or server uploads required.

Often you have a clean video recording without sound (or with background noise) and a separate high-quality voiceover, music track, or podcast audio that you need to merge into a single MP4 video file. Opening a heavy video editor just to replace or attach an audio track is slow and often forces hours of video re-encoding, ruining the original visual quality.

Instant Audio & Video Muxer solves this by performing stream-copy multiplexing directly on your computer or phone using WebAssembly and FFmpeg:

  • Zero Video Re-encoding (Lossless & Blazing Fast): The video stream is copied bit-for-bit directly into the new MP4 container in milliseconds. There is zero compression degradation and zero quality loss.
  • Automatic MP3 to M4A (AAC) Conversion: MP4 video containers standardly require AAC audio (.m4a) for smooth, universal playback. If you upload an .mp3 audio track, the tool automatically detects it and suggests converting it on-the-fly to high-quality M4A (AAC 192 kbps) right on the frontend before muxing. You can also download the converted .m4a file independently!
  • 100% Client-Side & Private: Your videos and audio files are never uploaded to any remote server or cloud. All processing happens entirely inside your local browser memory via FFmpeg WASM.
  • Supports Popular Media Formats: Works seamlessly with MP4, MOV, WebM, and MKV video files, and M4A, AAC, MP3, WAV, and OGG audio tracks.
  • Instant Preview & Download: Watch your combined video right in the browser player and download it with a single click.

Choose your video and audio track below to merge them instantly!

Instant Audio & Video Muxer Zero Re-encoding

Combine separate video and audio streams into a single MP4 file instantly right inside your browser.

Initializing WebAssembly engine...
FFmpeg WASM
1. Select Video File
MP4, MOV, WebM, or MKV
2. Select Audio Track
M4A, AAC, or MP3
✓ Converted to M4A: track.m4a Save .m4a
MP3 Audio Detected — Convert to M4A for Best Results?

MP4 video containers use M4A / AAC audio streams. Converting your MP3 to M4A in advance enables instant zero-reencoding muxing (< 1 second) and guarantees smooth audio playback on Apple, Android, Windows, and all modern browsers.

Processing...
✓ Video & Audio Successfully Combined!
Download Combined MP4
Fast lossless stream copy • 0% quality loss