Replace or mix
Replace removes the video's own sound and keeps only the music: the usual choice for holiday clips, product videos and slideshows where wind noise or chatter would only distract. Mix with music keeps both. Use the two volume sliders to put the music softly under speech, for example the video sound at 100% and the music at 20 to 30%.
Volumes go from 0 to 200%. When the two sounds together would get louder than a file can hold, the whole result is turned down a little, so nothing crackles or distorts.
Timing the music
Music starts at sets the moment in the video where the music begins; play the video, pause where you want it and press Use current time. Skip the start of the music jumps over a long intro, so the song begins with the part you like.
The result always has the length of the video. Music longer than the video is cut where the video ends; Fade out at the end lowers it gently over the last seconds instead of stopping abruptly. A song shorter than the video repeats if Repeat the music is ticked, otherwise the rest of the video stays silent or keeps only its own sound.
Quality, formats and privacy
The picture is copied without re-encoding, so it keeps its full quality and the work is quick even for long videos. Only the sound is encoded again: AAC in MP4 and MOV, Opus in WebM and MKV. The music can be MP3, M4A, AAC, WAV, OGG, Opus, FLAC or the sound of another video. Firefox cannot create AAC sound; for MP4 and MOV videos use Chrome, Edge or Safari. Nothing is uploaded: the video and the music stay on your device. Make sure you have the right to use the music, especially when you publish the video.