Add Audio to Video Online
Drop a video and an audio file - GrepCut replaces the soundtrack without re-encoding the picture. Nothing leaves your device.
If you already have a finished video and a separate voiceover, song, narration, or cleaned-up audio file, you do not need a full editor just to replace the soundtrack. GrepCut takes the video stream from your first file and the audio stream from your second file, then writes them into one MP4.
Your original soundtrack is not kept. The new audio file becomes the only audio track in the export. This is useful when your clip is silent, noisy, muted by a platform, or exported with the wrong audio take.
The video stream is copied when possible, so the picture is not re-compressed just because you changed the audio. If the video codec cannot be copied into MP4 cleanly, you may need a different workflow in the full editor.
Start with one video file. GrepCut is built around MP4, MOV, WebM, MKV, M4V, AVI, OGV, 3GP and MPEG inputs, plus other readable video files when the browser and FFmpeg can parse them.
Then add one audio file. MP3, WAV, AAC, M4A, FLAC, OGG and Opus are the tested audio inputs. The output is always an MP4, which makes the result easier to share, play, and import into common apps.
GrepCut trims the export to the shorter input so you do not get a long black tail, silent ending, or hidden extra audio after the picture ends. If your song is longer than the clip, the song is cut at the end of the video. If your voiceover is shorter than the clip, the exported video ends when the voiceover ends.
That behavior is intentional for a quick replacement tool. It keeps the result predictable without adding looping, padding, fades, or silence generation. For those timing choices, use the full timeline in GrepCut Studio.
Can you replace audio in an MP4 without rendering the video again?
Yes, when the video stream can be copied into the output MP4. GrepCut tries to copy the picture stream instead of re-encoding it, so changing the soundtrack does not automatically mean losing image quality.
Will your video or audio be uploaded?
No. GrepCut runs the audio swap locally in your browser using FFmpeg compiled to WebAssembly. Your files stay on your device.
Can you keep the original sound and add music on top?
Not in this quick tool. The replacement audio becomes the only soundtrack. Use [GrepCut Studio](/) if you need music, dialogue, and sound effects playing together.
What happens if your audio is longer than your video?
The export stops at the end of the shorter file. If your audio runs longer than the video, it is trimmed. If your audio is shorter, the video ends with it.
Can you sync audio that starts too early or too late?
This tool does not include offset controls. If your voiceover needs frame-level alignment, open the video in [GrepCut Studio](/) and move the audio on the timeline.
What formats can you add audio to?
You can start with MP4, MOV, WebM, MKV, M4V, AVI, OGV, 3GP or MPEG video. Your replacement audio can be MP3, WAV, AAC, M4A, FLAC, OGG or Opus. The downloaded file is MP4.
Why does GrepCut export MP4 instead of keeping the original container?
MP4 is widely supported by browsers, phones, editors, and social apps. Keeping one output container also makes the quick tool simpler and more predictable.
Will your file lose quality?
The picture should stay the same when stream copy works. The audio is taken from your replacement file and muxed into the output, so the result depends on the quality of the audio file you provide.