Guides5 min read
Music Ends Before Your Video? Fix the Length
Video cut short or music going silent after adding an audio track? The three length modes (video, shortest, loop) and which one to pick for your clip.
Add audio to video
Put music, narration, or any audio track onto your video — replace the original sound or mix over it. Free, private, nothing leaves your device.
Step 1
Drop the video here, or click to browse
MP4, MOV, WebM, MKV, or AVI — up to 2GB
Step 2
Drop the music or voiceover here, or click to browse
MP3, WAV, M4A, OGG, or FLAC — up to 2GB
A soundtrack tool that doesn't have to overwrite
Replace or mix — your call
One switch decides whether your track takes over the soundtrack or sits on top of what's already there. Almost no free online tool exposes that choice; it just overwrites.
The picture is never re-encoded
Your video's frames are copied into the new MP4 byte for byte. No generation loss, no softening, no waiting through a full transcode of footage you didn't change.
Volume from silent to double
Set the added track anywhere between 0% and 200% before it is written, so a faint voice memo can be lifted and a music bed can be tucked under dialogue.
Loop or pad a mismatched track
Short track under a long video? Loop it to fill the runtime, pad the remainder with silence, or end the file as soon as either one runs out.
Five audio formats in, MP4 out
MP3, WAV, M4A, OGG, and FLAC all go straight in — no converting your download to MP3 first — and come out as a 192 kbps AAC track inside a standard MP4.
Both files stay on your device
ffmpeg.wasm does the muxing inside your browser tab. Neither the video nor the track is uploaded, which matters when the audio is a client's unreleased music.
Two files in, one MP4 out
- 1
Drop in your video, then your audio
Two slots, one for each file: MP4, MOV, WebM, MKV, or AVI on the video side, MP3, WAV, M4A, OGG, or FLAC on the audio side. Either slot can be swapped later without touching the other.
- 2
Decide how the two tracks meet
Replace the video's original sound with your track, or mix your track over it. Set the added track's volume anywhere from silent to double, and choose what happens when the two lengths don't match.
- 3
Download the finished MP4
Play the result on the page before you commit to it, then download it. Both files were read on your device and never sent anywhere — there is no upload step in this tool at all.
What people add music to
Silent screen recordings, product demos that need a voice, slideshow exports, and clips whose original music has to go — all of it happens in the browser tab, with the picture copied across untouched.
A music bed under a silent screen recording
Screen capture tools frequently record no audio at all. Drop the recording in, pick a track, and loop it if the clip runs longer than the song — the result is a finished MP4 rather than a mute demo nobody watches to the end.
Narration over a product demo
Record the walkthrough separately on your phone, then lay it over the capture at 150% or so if your voice came out quiet. Mix mode keeps the app's own click and notification sounds audible underneath the narration.
Swapping a track before you re-upload
A video flagged for its background music can be rescued by replacing that audio with something licensed. Replace mode removes the original stream completely rather than layering over it, which is what a rights check is actually looking for.
Narrating a slideshow exported as video
Keynote and Google Slides exports arrive as silent MP4s. Adding a recorded voice track here turns one into a self-running explainer, with the length setting deciding whether the slides or the narration set the runtime.
Frequently asked questions
Can I keep the video's original sound instead of wiping it?
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Yes — that is what the Replace/Mix switch above the volume slider is for. Replace drops the original audio stream and keeps only your track. Mix runs both through ffmpeg's amix filter so the original stays audible underneath, with your track's level set independently by the volume slider. Most online tools only do the replace half and never say so.
Does adding music re-encode the video and cost me quality?
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No. The picture is stream-copied — ffmpeg runs with -c:v copy, so the exact same video packets are written into the new MP4 and nothing about the image is recalculated. That is also why a 1GB clip finishes in seconds instead of minutes. Only the audio is encoded, to AAC at 192 kbps. The one exception is a WebM whose VP9 picture MP4 cannot hold; that source is re-encoded at CRF 23, which is visually indistinguishable from the original.
My song is shorter than the video. What happens to the silent part?
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You choose. "Video" pads the rest of the timeline with silence so every frame survives. "Loop" restarts the track from the top as many times as it takes to reach the end — handy when a 30-second music bed has to cover a four-minute screen recording. "Shortest" does the opposite and cuts the video off where the track runs out.
What if my music is longer than the clip?
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With "Video" selected the track is trimmed at the last frame, so the file ends when the picture does. "Shortest" behaves the same way in this direction. Nothing fades on its own — the cut is hard, so pick a track whose ending lands where you want it, or trim the audio first.
Why does the volume go above 100%?
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Because a quiet phone voiceover often needs help. 100% leaves your track at its recorded level and 200% is roughly a 6 dB boost, applied through ffmpeg's volume filter before the mix. It works the other way too: mixing a music bed under someone talking usually sounds right somewhere between 25% and 40%, not at full level.
Which audio and video formats can I combine?
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Video in: MP4, MOV, WebM, MKV, AVI. Audio in: MP3, WAV, M4A, OGG, FLAC. Any combination of the two works — an M4A voice memo onto an MKV screen capture is as valid as an MP3 onto an MP4. The output is always MP4 with an AAC track, which is what social platforms, editors, and phones all accept.
What happens if the video has no sound at all and I choose Mix?
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The tool checks the source for an audio stream before it starts. If there isn't one — common with screen recordings, drone footage, and anything already muted — there is nothing to mix with, so your track is added on its own and the result screen tells you that happened. You never get a file labelled as a mix that only contains one track.
I need to take audio off a video instead. Is there a tool for that?
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Yes — PixelTools' Remove Audio from Video tool is the inverse of this one and strips the audio stream out entirely. A common pairing is to mute a clip there first when you want a guaranteed-clean starting point, then bring the muted file back here to lay your licensed track over it.
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