How to add audio to a video
- Choose the video and the audio file together, or add them one after the other. The order doesn’t matter: the tool recognises which is which.
- Press Add audio to video.
- Download the MP4 with the new soundtrack.
Common uses
- Add background music to a slideshow or a silent clip.
- Replace noisy or distracting sound with music.
- Put a recorded voiceover on a screen recording or tutorial.
- Swap the audio of a clip for a cleaner recording of the same moment.
Using music legally
Music on social media is checked for copyright. Use tracks you own, royalty-free music, or music the platform provides. Many sites offer free music libraries, including YouTube’s Audio Library.
What you get
| Part | In the new MP4 |
|---|---|
| Picture | Copied unchanged from H.264 videos; others re-encoded to H.264 |
| Sound | Your audio file, as AAC, cut to the video’s length |
| Original sound | Removed |
| Length | The video’s length |
The note under the result tells you if the audio was shorter than the video, so you know the end is silent.
Preparing the audio
- Voiceovers: record them in a quiet room, close to the microphone. A phone’s voice recorder app works well; iPhone Voice Memos files can be used directly (they’re M4A).
- Music: if the track is longer than the video, it simply stops when the video ends. For a neat ending, choose a track about the video’s length or one that fades out.
- Timing: the audio always starts at the beginning of the video. To start it later, add silence to the start of the audio file in an audio editor first.
Privacy
Both files are combined inside your browser using Mediabunny, an open-source media library. Nothing is uploaded.
Questions
Is the original sound kept?
No. The audio file replaces the video's own sound. Mixing music under the original sound, for example under someone speaking, isn't supported yet.
What if the audio is longer or shorter than the video?
Longer audio is cut off where the video ends. Shorter audio plays from the start and the rest of the video is silent. Trim the audio or the video first if you want them to end together.
Which audio files can I use?
MP3, M4A, AAC, WAV, FLAC, OGG and Opus files. The sound is saved as AAC inside the MP4, the standard for MP4 videos; AAC files are copied without re-encoding.
Is the video re-encoded?
Not if it's H.264, as most phone and camera videos are. The picture is copied unchanged, so adding audio is fast and lossless. Other video formats are converted to H.264.
Last reviewed 6 November 2026