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Use FFmpeg’s amix filter to combine the video’s existing audio with a separate music file, then map the original video stream and encode the new audio. This preserves speech or ambience while adding a music bed. The example below assumes input.mp4 has an audio stream and music.mp3 is the track you have permission to use.
Mix the original audio and music with FFmpeg
Open a terminal in the folder containing both files and run:
ffmpeg -i input.mp4 -i music.mp3 -filter_complex "[1:a:0]volume=0.25[music];[0:a:0][music]amix=inputs=2:duration=first:dropout_transition=0[mix]" -map 0:v:0 -map "[mix]" -c:v copy -c:a aac -b:a 192k output.mp4This is an illustrative command based on FFmpeg’s documented filter behavior, not a tested recipe. The volume=0.25 setting lowers the music input before mixing; it is a starting point, not a universal voice-to-music level. Listen to the result and adjust it to keep speech intelligible and avoid clipping.
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-map 0:v:0keeps the video’s first video stream.-map "[mix]"sends the filtered mix to the output as its audio stream.-c:v copycopies the video without re-encoding it, provided the source codec is compatible with the MP4 output.-c:a aac -b:a 192kencodes the filtered audio as AAC at the specified bitrate. Filtered audio has to be encoded as a new stream.
FFmpeg’s Cookbook example, updated September 27, 2026, illustrates the general arrangement of mixing audio, mapping the video separately, and encoding the filtered audio. Check your own streams and output rather than assuming the example fits every file.
Choose which audio stream controls the mix duration
The command uses duration=first, so the mixed audio ends when the first audio input—the video’s original audio—ends. Change the option to match what should happen at the end:
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| Option | When the mix ends | Use it when |
|---|---|---|
duration=first |
When the first audio input ends | The original video audio should determine the mix length. |
duration=shortest |
When the shorter audio input ends | You do not want the mix to outlast whichever audio stream is shorter. |
duration=longest |
When the longer audio input ends | The longer audio stream should continue after the other ends. |
The video stream may end before the mixed audio, depending on the input lengths and chosen duration. If you want the overall output to stop with the shortest stream, consider adding -shortest before the output filename, then check the exported file’s ending. If you want music to continue after the original audio stops, choose a duration strategy that allows that and verify the result.
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dropout_transition controls the time used to renormalize volume when an input ends; FFmpeg documents a default of two seconds. The example sets it to zero to avoid that transition. Remove the option or select a different value if you want a transition when one input ends. amix normalization is enabled by default. Disabling it can cause heavy clipping if you have not managed the input levels.
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Check the inputs and the exported file
- Confirm the video has audio. The example expects a first audio stream at
[0:a:0]. If the video has no audio, there is nothing to preserve; mix and map the music stream instead. - Confirm the stream indices. Files can contain multiple audio or video streams. The example selects the first audio stream from each input and the first video stream from the video file. Adjust the indices if the streams you need are elsewhere.
- Listen to the output. Check speech intelligibility, music level, clipping, and the point where the mix ends. Lower the music further if it competes with dialogue.
- Check playback compatibility. The command writes an MP4 with copied video and newly encoded AAC audio. If the source video codec is not suitable for the intended MP4 playback, re-encode the video with an appropriate codec instead of using
-c:v copy.
Use music you can publish on YouTube
Adding music technically does not grant permission to use it. YouTube says tracks and sound effects in its Audio Library are royalty-free production music, and that the library is the source whose tracks YouTube knows to be copyright-safe. Some Audio Library tracks use Creative Commons licenses and require attribution in the video description. Check the selected track’s terms and include any required credit.
YouTube does not give legal advice and says it is not responsible for issues with music from other libraries. A “royalty-free” label from an external source is not, by itself, YouTube’s assurance that a track is safe for your upload. Review the license for the specific track and confirm that it permits your intended YouTube use. See YouTube’s Audio Library guidance.
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