How to drop background music under your voiceover across an entire folder of videos in seconds — free, on your own computer, using Claude + a free tool called ffmpeg.
Normally, adding music means opening each video in an editor, uploading it, dragging a music track, lining it up, exporting, downloading. One at a time. For a library of videos that is hours of work. Here is the difference.
Open an editor → upload each video → drag music → line it up → export → download. Repeat for every single video. Big files take forever to upload.
Point Claude at a folder, say "add this music." It does every video at once, a few seconds each, on your computer. No uploading, no quality loss, free.
This is the whole secret. If you have these two, you are ready. Setup is one time only.
This is Claude that lives on your computer and can actually work with your files — not the chat on the website. It is the "hands" that does the work for you.
The regular Claude/ChatGPT website cannot touch your video files. You need the desktop version.
A free, open-source tool that is the actual workhorse for editing audio and video. It is the engine quietly running inside tons of apps you already use. Claude drives it for you — you never type a command.
Free forever. No account, no subscription, no upload limits.
You do this once. After that, every future batch of videos is just one sentence to Claude.
Go to claude.ai and download the Claude desktop app for Mac or Windows (or the VS Code extension if you use VS Code). Open it and sign in with your Claude account.
This is the version that can open folders and run tools on your machine.
You do not have to figure this out yourself. Just open Claude Code and type:
Claude will handle it. If you ever want to do it yourself, it is one line:
winget install Gyan.FFmpegbrew install ffmpegNo menus, no timeline, no exporting. Three simple moves.
Drop all the videos you want music on into a single folder. Put your music file (a .mp3 or .wav) somewhere you can find it too.
Copy this, swap in your folder and music, and send it:
“Add background music to all the videos in this folder: [your folder]. Use this music file: [your music file]. Keep the music at 10% volume under my voice with a 5-second fade-out at the end. Keep my original videos backed up.”
Tip: you can usually drag a folder right into Claude to get its location, or right-click the folder → Copy as path.
Claude works through every video in the folder — a few seconds each — mixing your music underneath your voice, looping it to fit the full length, and fading it out at the end. Your originals are kept safe in a backup folder, so you always have the no-music versions too.
Everything else is automatic. Just tell Claude different numbers and it adjusts.
How loud the music is under your voice. Lower = quieter. 10% is a gentle bed that keeps narration crystal clear. Want it softer? Say 8%. More present? Say 15%.
How the music ends. A 5-second fade-out lands softly instead of cutting off. Want a longer tail? Ask for 8 seconds.
You never need to see this — Claude writes and runs it for you. But if someone technical asks "what's actually happening?", here is the one command behind it all.
ffmpeg -i "video.mp4" -stream_loop -1 -i "music.mp3" \ -filter_complex "[1:a]volume=0.10,afade=t=out:st=END-5:d=5[bg];\ [0:a][bg]amix=inputs=2:duration=first:normalize=0[a]" \ -map 0:v -map "[a]" -c:v copy -c:a aac -b:a 192k -shortest "out.mp4"
volume=0.10 = music at 10% · afade ... d=5 = 5-second fade ·
normalize=0 = keep the voice at full volume · -c:v copy = don't re-render the video (fast + lossless) ·
-stream_loop -1 + -shortest = loop the music and trim it to the video length.