Add audio to a video

How to use it#

1. Drop your video on the first box and the sound on the second, or both on the first at once. The sound can be an MP3, M4A, WAV, or another video.
2. Choose whether the new sound replaces the video's own or mixes with it, and set the levels.
3. The video always plays from its start. To have the sound come in later, set Sound starts at; to start partway into the sound, set Skip into the sound. Play either one and press From video or From sound to take the time from where it is.
4. The video is made again each time a setting changes. Play it in the preview, then download it.

Replace the sound, or mix it in#

Replace it when the video's own sound is noise you do not want, such as wind, traffic, or a room full of talk, or when it has none. Mix them when the video has speech or sound worth keeping and the music should sit under it. Music at about 20% to 30% of its level keeps a voice clear over it. At 100% it plays as loud as it was recorded, which usually buries the voice.

A voiceover recorded on its own, such as an M4A from Voice Memos on an iPhone, goes on the same way: replace the sound to hear only the voice, or mix it over the video's own sound turned down low. Each level runs from 0% to 200%, so a quiet recording can be raised as well as a loud one lowered.

Either way the picture is copied into the new file as it is, frame for frame, so it looks exactly as it did and is not made again. Only the sound is new. When the sound file is AAC or MP3 and nothing about it changes, it is copied in as well, which is fastest and loses nothing. A level, a fade, a loop, a start time, or a mix means the sound is decoded and encoded again as AAC at 192 kbps, which is very hard to tell from the original.

When the music is shorter or longer than the video#

A song longer than the video is cut where the video ends. Turn on the fade and it falls away over the last two seconds instead of stopping mid-note. To use a different part of the song, such as its chorus, set Skip into the sound to where that part starts.

A song shorter than the video can loop: it starts again from where it began until the video ends. Or let it end, and the rest of the video plays with no added sound. When the video's own sound is mixed in, that carries on to the end either way.

Which formats work#

The video can be an MP4, MOV, WebM, or MKV of up to 2 GB, and the result keeps its format. The sound can be an MP3, M4A, AAC, WAV, OGG, Opus, or FLAC file, or another video, whose sound is used. To keep only the sound of a video as a file of its own, use MP4 to MP3.

A WebM holds only Opus or Vorbis sound, so any other sound added to a WebM is encoded as Opus. An MP4 or MOV gets AAC, which every phone, computer, and editing app plays.