Add Audio to Video Online Free

Why Combining Audio and Video Is a Core Skill

Almost every video you watch has an audio track that someone chose. A YouTube tutorial has narration. A product demo has background music. A TikTok has a trending sound. A corporate video has a polished voiceover. Some of those tracks were recorded at the same time as the video. Many were not. The skill of merging a separate audio track with a video clip is one of the most common tasks in media work, and it is the reason this tool exists.

Traditionally, merging audio and video required a desktop editor like Adobe Premiere, Final Cut Pro, or DaVinci Resolve. Those tools are powerful, but they cost money, demand installation, and take hours to learn. For anyone who needs to add a single music track to a single clip, that overhead is absurd. A browser-based mixer solves the job in under a minute and keeps your files on your own machine.

This page explains how the tool works, when to use which mode, how to choose bitrates, and how to avoid the common mistakes that ruin merged audio.

How This Tool Works

The tool decodes the video and audio files locally in your browser. It reads frames from the video, mixes the audio tracks according to your settings, and captures the combined result with the MediaRecorder API. The export is a WebM file with VP8 or VP9 video and Opus audio.

Everything runs on your device. No file uploads to a server, no cloud storage, no waiting for a remote render queue. The merged file never leaves your machine. That matters for client footage, confidential interviews, or personal videos you would rather not send to a third party.

The WebM output plays in every modern browser and most social platforms. If you need MP4 specifically, convert the WebM afterward with a video converter. WebM is widely accepted by YouTube, Vimeo, and most CMS platforms. Social networks like Instagram and TikTok accept MP4 uploads but often transcode internally; WebM uploads usually work but are sometimes rejected. In those cases, convert first.

Add, Replace, or Mix — Which Mode Fits Your Task?

Add mode

Use Add mode when you want a new audio track on top of the original sound. The original audio stays audible, and the new audio plays over it. This is the right choice for background music under dialogue, ambient sound over a silent clip, or a sound effect that plays once during the video.

Add mode uses a preset volume balance so both tracks are audible. For finer control, switch to Mix Both and set each volume manually.

Replace mode

Use Replace mode when the original audio is wrong. Maybe the recording has noise. Maybe the speaker misspoke. Maybe you recorded a voiceover separately and want it to be the only soundtrack. Replace mode mutes the original audio entirely and uses only the new track.

Replace is the most common mode for tutorials, explainer videos, and product demos. It is the cleanest way to guarantee the audio you hear is the audio you intended.

Mix Both mode

Use Mix Both when you need precise control over the balance. Set the original video audio to 20 percent and the new music to 80 percent for a subtle dialogue bed. Set the original to 100 percent and the new audio to 40 percent to keep speech dominant. Mix Both is the right choice for any professional-style edit where the balance between tracks matters.

Step-by-Step: Merge Audio with Video

1. Upload your video. Drag an MP4, WebM, MOV, AVI, MKV, FLV, or WMV file into the left upload card. The tool decodes the video and shows the resolution and duration.
2. Upload your audio. Drag an MP3, WAV, OGG, AAC, FLAC, or M4A file into the right card. You can also drop a video file with an audio track; the tool extracts its audio automatically.
3. Pick a mode. Choose Add, Replace, or Mix Both. The mode decides which audio plays in the export.
4. Adjust volume. Set the new audio volume between 0 and 200 percent. Set the original volume if you chose Mix Both.
5. Set the offset. If the audio should start after the video begins, set the offset in seconds. Useful for music that should wait for a spoken intro.
6. Add fades. Use Fade In and Fade Out to smoothly raise or lower the new audio at the start and end.
7. Choose output format and bitrate. WebM VP8 for maximum compatibility or VP9 for better quality at similar file size. Video bitrate from 1 to 30 Mbps. Audio bitrate from 64 to 320 kbps.
8. Enable loop or trim. If the audio is shorter than the video, enable Loop to repeat it or Trim to cut the video at the audio's endpoint.
9. Click Merge. The tool processes frames and audio in real time, shows progress, and produces a download when it finishes.
10. Download. Save the exported WebM file. No watermark is added.

Understanding the Controls

Audio Volume

Controls the new audio's level. 100 percent is the source level. 200 percent amplifies it, but audio above 100 percent can clip. If the new track is quiet, keep it at 150 to 180 percent rather than pushing to 200. If the track is loud and needs to sit under dialogue, drop it to 20 or 30 percent.

Original Video Audio

Controls the video's native audio. At 0 percent, the original audio is muted entirely. At 100 percent, it plays at source level. In Replace mode, this is fixed at 0. In Mix Both, you control it directly.

Audio Start Offset

Delays the new audio by N seconds. If you want a spoken intro to finish before the music starts, set an offset of 5 seconds. The tool keeps the video playing during the offset and starts the new audio at the right moment.

Fade In / Fade Out

Ramps the new audio up or down over the given number of seconds. A 0.5-second fade in at the start prevents the abrupt jump from silence to music. A 1-second fade out at the end gives the audio a natural close.

Video Bitrate

The number of megabits per second used for the video track. Higher bitrate preserves more detail and produces larger files. For 1080p content, 6 to 10 Mbps is fine. For 4K, aim for 20 to 30 Mbps. Lower bitrates save file size and speed up export, but fine detail may soften.

Audio Output Bitrate

The Opus bitrate for the audio track. 64 kbps is enough for pure voice. 128 kbps is fine for standard music. 192 kbps is high quality. 256 and 320 kbps are for studio-grade or archival work.

Choosing the Right Bitrate for Your Use Case

Use Case	Video Bitrate	Audio Bitrate
Quick draft or preview	2 Mbps	64 kbps
Social media (TikTok, Reels)	4 Mbps	128 kbps
YouTube / general web	8 Mbps	192 kbps
Product demo / tutorial	10 Mbps	192 kbps
Client delivery / short film	15–20 Mbps	256 kbps
Archival / master export	25–30 Mbps	320 kbps

These are starting points. Every project has different priorities. A voice-heavy tutorial with static visuals can drop to 4 Mbps without visible loss. A high-motion sports clip needs more.

Real-World Use Cases

Tutorials and explainers

Record the screen silently, then add a voiceover in Replace mode. This workflow produces cleaner audio than speaking while recording because the microphone is not picking up keyboard clicks or fan noise. Most professional tutorial channels use this technique.

Product demos

Add ambient background music under the product's original sound using Mix Both. Set the original audio to 60 percent and the new music to 30 percent for a subtle bed that does not distract from the presenter.

Social media clips

Replace the audio with a trending track before posting to TikTok or Reels. Use Trim audio to video length so the new track does not extend past the clip.

Podcast video versions

If your podcast has an audio-only master, use Replace mode to add the mastered audio to a video version recorded on camera. Both stay in sync.

Fixing broken audio

Sometimes video files arrive with no audio at all. Merge the original recording's audio back in. This is common when re-encoding corrupted files or when a clip was exported without the audio track by mistake.

Accessibility and dubbing

Add a translated voiceover on top of the original at low volume so listeners can hear both languages. This is a common dubbing technique for reaching international audiences.

Family and personal videos

Wedding videos, birthday clips, and travel footage often sound better with a curated music track. Replace the noisy original with a well-chosen song. Trim the video to match the music's length.

Quality: What to Watch For

Merged audio and video can degrade in three ways. Watch for each.

Sync drift. If the audio and video are not perfectly in sync, the viewer notices within a second. The tool processes in real time, so sync should stay tight. If your source files have different frame rates or sample rates, drift can happen. Test a short clip first.

Audio clipping. Setting the new audio volume above 100 percent risks clipping the waveform. If the output sounds distorted, drop the volume to 100 percent or lower and re-export. Clipping cannot be undone after export.

Compression artifacts. Re-encoding video always loses some information. Choose a bitrate high enough for the content type. Low bitrates produce blocky motion in high-motion scenes and blurred fine text in screen recordings.

Privacy: What Happens to Your Files

The tool never uploads your files. Video and audio are decoded, mixed, and encoded entirely in your browser using the Web Audio API, Canvas API, and MediaRecorder API. When you close the tab, the files leave memory.

That matters for recordings of client work, private events, or unreleased content. Server-based mixers require trust in the host. This tool removes that requirement.

Tips for Better Merges

• Preview before you merge. If you can listen to the audio and video separately first, do it. Confirm the audio's start point and length before processing.
• Normalize before uploading. If your audio is quieter or louder than expected, run it through the audio compressor first. That prevents volume surprises in the export.
• Trim silence first. Use the audio silence remover on the new audio to cut dead air. It shortens the file and makes the timing predictable.
• Loop for continuous background music. If the new track is shorter than the video, enable Loop. That produces a continuous bed without gaps.
• Match frame rate. If the video is 30fps and your audio's original video was 24fps, the timing may drift. Test a short clip first.
• Use Replace mode for voiceover. If you recorded narration separately, Replace is the cleanest mode. It removes the original audio entirely and uses only the voiceover.
• Convert to MP4 afterward if needed. WebM works in most places, but Instagram, TikTok, and some CMS platforms prefer MP4. Use the video converter after export if required.
• Compress the final file. If the exported WebM is too large for its target, run it through the video compressor at a lower bitrate.

Related Video and Audio Tools on VideoTools

Merging is one step in a longer media workflow. These tools cover the rest, all in the same browser-based, no-upload environment:

• MP3 Audio Compressor — reduce audio size before merging
• Audio Silence Remover — trim dead air from the new track
• Audio Speed Changer — adjust the audio's duration to match the video
• Extract Audio — pull the original soundtrack out as a separate file
• Video Converter — convert the merged WebM to MP4 or MOV
• Compress Video — shrink the final file for upload
• Add Text to Video — add captions or titles to the merged result
• Add Image to Video — overlay logos or images on the final clip

Frequently Asked Questions