Adding background music to a video is one of the most common post-production tasks. A travel montage needs an atmospheric track. A product demo needs subtle background music. A birthday compilation needs a celebratory song. A real estate walkthrough needs something pleasant that is not distracting.
Traditional video editing software (Premiere Pro, DaVinci Resolve, Final Cut) can do this, but importing a project, dragging clips to a timeline, adjusting audio levels, and exporting is a 15-minute process for a task that should take 30 seconds.
What "adding audio" actually does technically
A video file is a container (MP4, MKV, WEBM) that holds multiple streams. When you add audio to a video, the tool places the audio stream alongside the video stream inside the container. The video data is not changed. The audio data is not changed. They are simply packaged together.
If the video already has an audio track (ambient sound, narration), you have two options:
Replace: Remove the existing audio and insert the new audio. The original sound is gone entirely. This is what you want when the original audio is unusable (wind noise, background chatter).
Mix: Combine the existing audio with the new audio so both play simultaneously. The original sound continues while music plays underneath. This requires audio mixing, which adjusts the relative volume of each track.
For most "add music" tasks, replacing is simpler and produces cleaner results, especially if you have already muted the video.
Audio and video duration alignment
When the music track and video have different durations, the tool must decide what to do:
Music longer than video: The output matches the video duration. The music fades or cuts when the video ends. Extra music is discarded.
Music shorter than video: The output matches the shorter track. Some tools truncate the video to the music length. Others pad the remaining video with silence. Others loop the music.
For best results, choose a music track that matches or exceeds your video duration. If the music is slightly long, the cut at the end is usually fine for social media. If the music is significantly short, consider looping it before adding it to the video.
Volume levels
When adding music to a video with no existing audio (muted), the music plays at its native volume. This is usually fine if the music track was mastered at a reasonable level.
When mixing music with existing audio, the relative volume matters enormously. Industry standard for background music under speech is -18 to -24 dB relative to the speech track. This means the music should be roughly 6-10 times quieter than the speech. If the music is too loud, it competes with the speaker. If too quiet, it adds nothing.
Since MediaBrew's add audio tool replaces (not mixes) the audio, volume balancing is not a concern. The music plays at its full level as the only audio track.
Copyright and licensing
Adding copyrighted music to video you share publicly will result in copyright strikes, content removal, or audio muting on every major platform. Use royalty-free music from sources like the YouTube Audio Library, Pixabay Music, or Uppbeat. Check the license for each track: some require attribution in the video description, some do not. The MediaBrew add audio tool combines your video and music file into a single output, replacing the existing audio entirely. Processing runs locally via FFmpeg WebAssembly.