Use FFmpeg’s amix filter to combine a video’s existing audio with a separate music file while keeping the video stream intact when its codec and container allow it. Lower the music before mixing, choose which audio input controls the mix’s duration, then listen for clear speech and clipping before uploading.
Mix the original audio and music with FFmpeg
The command below keeps the video stream from input.mp4, mixes its first audio stream with music.mp3, and encodes the new audio as AAC:
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.mp4Here, volume=0.25 reduces the music input before the mix. It is a starting point, not a universal voice-to-music setting: the right level depends on the source recording, music, and intended prominence. Listen to the exported file and adjust as needed.
amix combines multiple audio inputs into one output. The video is mapped separately with -map 0:v:0; the filtered audio is mapped as [mix]. Because filtering creates a new audio stream, it must be encoded rather than stream-copied. The video can be copied with -c:v copy when its existing codec is compatible with the output container. See the FFmpeg filter documentation and this FFmpeg Cookbook example (updated September 27, 2026, for FFmpeg 8.1) for the documented filter behavior and a published command pattern.
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Choose how long the audio mix should run
The command uses duration=first, so the first audio input—the video’s original audio—sets the mixed-audio duration. FFmpeg also documents these alternatives:
| Setting | When the mixed audio ends | Use it when |
|---|---|---|
duration=first |
When the first audio input ends | The original audio should control the mix duration. |
duration=shortest |
When the shortest audio input ends | You do not want the mix to outlast whichever audio stream ends first. |
duration=longest |
When the longest audio input ends | The longer audio stream should continue after the other ends. |
Audio duration and video duration are separate considerations. If the video ends before the mixed audio, consider output-level duration handling such as -shortest, then check the exported file to confirm it ends as intended. The dropout_transition option controls how long volume renormalization takes when an input ends; its documented default is 2 seconds. The example sets it to 0 to avoid that transition.
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Adjust levels and check the result
amix normalization is enabled by default. If you disable it, FFmpeg warns that the result can clip heavily unless input levels are controlled. Avoid assuming that one music level will work for every recording.
- Play the output and check that speech or other original sound is understandable beneath the music.
- Listen for distortion or harsh peaks that may indicate clipping; reduce input levels and render again if needed.
- Check the beginning and end, especially if one input ends before the other, and confirm that the picture and audio stop when you expect.
Check streams and file compatibility
The example assumes the video has an audio stream at 0:a:0 and the music has one at 1:a:0. If the source file has no audio, there is no original sound to preserve: mix the music input by itself and map that filtered result. Check the actual stream indices if a file contains multiple audio tracks, and confirm that the output container supports the chosen video and audio codecs.
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Use music you are allowed to publish
Before uploading, check the track’s license and any attribution requirement. YouTube says its Audio Library in YouTube Studio provides royalty-free production music and sound effects, and that tracks from this library are the only ones it knows to be copyright-safe. Its guidance says Creative Commons tracks require attribution in the video description. A “royalty-free” label from another library is not YouTube’s assurance; review that source’s terms. YouTube also says it cannot give legal advice or take responsibility for issues with other libraries. See YouTube’s Audio Library guidance.
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Mix and export your video locally with FFmpeg, then use StreamNeo if you want uploaded prerecorded videos to keep a YouTube channel live around the clock. Upload a recording or build a playlist, add your YouTube stream key once, and go live. StreamNeo loops uploaded videos from the cloud; it does not broadcast a camera feed. Nothing has to stay on at home, and the same flat price per slot covers any uploaded quality up to 4K 60fps without re-encoding or quality tiers. It can automatically recover if YouTube drops the stream. The first day is free with no card; after that, the monthly option is $9.99 per month. See StreamNeo or its pricing, then start your free day.
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