How to Mux Audio and Video into MKV

Combining separate audio and video tracks into a single, cohesive file is a process known as multiplexing, or “muxing.” This article provides a straightforward guide on how to merge these streams into a high-quality MKV (Matroska) container using two of the most popular and free tools available: FFmpeg and MKVToolNix. By following these steps, you can combine your files quickly without losing any video or audio quality through re-encoding.

Method 1: Using FFmpeg (Command Line)

FFmpeg is a powerful command-line tool that can mux audio and video streams instantly because it copies the streams directly without re-encoding them.

1. Download and Install FFmpeg: Ensure FFmpeg is installed on your system and added to your system’s environmental variables (PATH).
2. Open Command Prompt or Terminal: Navigate to the folder where your audio and video files are stored.
3. Run the Muxing Command: Enter the following command, replacing the placeholder names with your actual file names:

ffmpeg -i video.mp4 -i audio.wav -c copy output.mkv

• -i video.mp4: Specifies the input video file.
• -i audio.wav: Specifies the input audio file (this can also be MP3, AAC, M4A, etc.).
• -c copy: Instructs FFmpeg to stream-copy both the audio and video tracks. This prevents re-encoding, preserving original quality and finishing the process in seconds.
• output.mkv: The name of your final muxed MKV file.

Method 2: Using MKVToolNix (Graphical Interface)

If you prefer a visual interface, MKVToolNix is the industry-standard software designed specifically for creating and modifying MKV files.

1. Download and Open MKVToolNix: Install and launch the MKVToolNix GUI application.
2. Add Source Files: Drag and drop your separate video and audio files into the “Source files” section, or right-click and select “Add files.”
3. Select Tracks: In the “Tracks, chapters and tags” section below, make sure the checkboxes for both the video track and the audio track you want to merge are checked. You can also click on individual tracks to set the language, track name, or default track flags in the properties panel on the right.
4. Set Output Destination: At the bottom of the window, choose where you want to save your new file in the “Destination file” field.
5. Start Multiplexing: Click the “Start multiplexing” button at the bottom of the window. The tool will merge the streams into a single MKV container within seconds.