multimux: Master Audio Mixdown Suite

A lightweight, cross-platform desktop app (Electron, SolidJS, TypeScript) to visually mix multiple discrete audio tracks from a recording into one master track while preserving video bit-for-bit (-c:v copy).

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Table of Contents

1. About The Project
   • Built With
2. Getting Started
3. Usage
4. Explanation
5. Changelog
6. Contributing
7. Contact

About The Project

Screen-recording apps often capture mic, game, and chat on separate tracks. Players usually only hear track 1. multimux mixes selected tracks with FFmpeg amix while copying the video stream losslessly.

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Built With

• Desktop —
  • 
  • 
• Media — / FFprobe

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Getting Started

Prerequisites

• Windows 10/11, macOS, or Linux
• ffmpeg and ffprobe on PATH
• Node.js + pnpm

Installation

pnpm install
pnpm run dev

Use F12 for the developer console if needed.

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Usage

First mixdown

1. Launch multimux.
2. Drag a recording onto the Inlet Port.
3. Toggle channels and adjust faders.
4. Click Mix & Mux Master.
5. Open the output from the completion overlay.

Volume (dB)

• Drag faders up to +6.0 dB or down to mute.
• Double-click a fader to reset to 0.0 dB.

Codecs

Under Output Mux Specs, choose AAC or OPUS and a bitrate (128k–320k).

Silent video

Mute all channels, then mix — multimux uses FFmpeg -an.

Build scripts

• pnpm run dev — hot-reload development
• pnpm run build — compile assets
• pnpm run build:win — Windows installer under dist/

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Explanation

Unlike full re-encodes, multimux:

1. Video stream copy (-c:v copy) — lossless, fast container remux of video packets.
2. Audio mixdown (amix) — decode selected tracks, apply volume, mix, encode one stereo master.

Example filter graph (CH1 at 1.0, CH3 at 1.5):

[0:a:0]volume=1.0[a0]; [0:a:2]volume=1.5[a2]; [a0][a2]amix=inputs=2:duration=longest:dropout_transition=0[a]

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Changelog

See CHANGELOG.adoc.

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Contributing

Fork, branch, and open a pull request.

Top contributors

For per-person profile links, prefer all-contributors.

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Contact

Ryan Johnson — @amdphreak

Project Link: https://github.com/AMDphreak/multimux

Site: https://ryanjohnson.dev

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