Dubbing - Replace soundtrack with new audio using API

Your users want to swap voiceovers, add new languages, or drop in an AI narration.
You just want to give them a button.

But behind that button is hours of work: stripping the original audio, aligning the new track, transcoding the file, preserving sync - all while supporting different formats and keeping quality intact.

If you're building a video product - whether it’s an OTT platform with regional dubs, an AI dubbing tool, or a CMS that updates training content - replacing a video’s audio shouldn’t be a post-production job. It should be a clean, automated feature. Something your users trigger, and your backend handles it all programmatically.

That’s exactly what the FastPix API does. In this guide, we’ll show you how to programmatically replace audio tracks. Whether you’re updating one video or thousands, you'll learn how to make audio replacement a native part of your app, not a workflow bolted on after the fact.

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How audio replacement works

Before you call the API, it helps to understand what’s happening under the hood.

Replacing a video’s audio isn’t just about muting one track and dropping in another. To do it properly when a video is uploaded to your application, especially at scale - you need to:

• Remove the original audio stream without corrupting the container
• Insert the new audio track while preserving sync
• Handle variations in codec, duration, and format (e.g., MP4, MOV, MKV)
• Normalize and clean the new audio to avoid jarring playback
• Re-package the video in a way that works across all devices and players

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Traditionally, this requires manual editing tools or custom FFmpeg scripts - but those approaches break at scale. That’s where FastPix comes in.

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FastPix is a full-stack video API platform built for developers who are tired of stitching together complex video workflows from scratch. Instead of juggling tools like FFmpeg, storage layers, and encoding pipelines, FastPix lets you do everything - upload, transform, analyze, stream, protect - through a single API.

It’s designed for real products, not prototypes. If you’re building anything from a short video app to a podcast tool to any product that involves video, FastPix helps you ship media features faster - without wrangling low-level video engineering.

The FastPix API abstracts all the complexity with audio replacement (dubbing) into a simple workflow: You pass in a video file, a new audio track, and the API returns a synced, production-ready video - with no manual alignment, transcoding, or error-prone guesswork. It also optimizes audio loudness, integrates cleanly into your pipelines, and saves hours of manual effort - whether you’re updating one video or ten thousand.

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Replace existing audio with new tracks using FastPix API

FastPix simplifies the process of replacing a video’s audio track. With just two inputs, you can update the audio seamlessly, leaving the complexities of synchronization and integration to the API. Here's a clear guide to how it works:

Step 1: Specify the video

Provide the URL of the video where the audio needs to be replaced. This URL serves as the input video for the API call.

Json

1{
2    "type": "video",
3    "url": " http://commondatastorage.googleapis.com/codeskulptor-assets/Collision8-Bit.ogg"
4}

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Step 2: Add the new AudioTrack

Provide the URL of the new audio file that will replace the existing track.The API merges the new audio with the video while ensuring frame-perfectsynchronization.

Example API input:

Json

{
    "type": "audio",
    "swapTrackUrl": "http://commondatastorage.googleapis.com/codeskulptor-assets/Collision8-Bit.ogg"
}

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Step 3: Process the request

Once both inputs are provided, FastPix API handles the heavy lifting:

• Ensuring the audio is perfectly synchronized with the video.
• Maintaining consistent audio and video quality.
• Processing the output efficiently for instant playback or download.

This streamlined process eliminates the manual challenges of audio replacement, making it ideal for both individual edits and bulk workflows.

For detailed instructions refer to the FastPix API Documentation.

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Real-World use cases for audio replacement

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Update corporate training videos with ease

Keep training materials fresh without starting from scratch. When policies change or new procedures roll out, simply replace outdated voiceovers with updated ones. Need multilingual versions? Swap audio tracks in different languages effortlessly to ensure global teams stay aligned. This saves both time and resources, keeping everyone up to speed.

Localize marketing campaigns seamlessly

Expand your campaign’s reach with localized voiceovers. Instead of re-editing videos for each region, replace the original English audio with translated versions. Deliver tailored messages to diverse markets while preserving the polished visuals that capture attention. The result? Greater engagement and cultural connection with minimal effort.

Refresh E-Learning content in no time

Course material evolves, but re-recording video content every time shouldn’t be part of the process. Swap outdated narrations with new, accurate audio to keep lessons current and engaging. Whether it’s a quick update or a complete overhaul, the API makes maintaining e-learning modules hassle-free.

Make content accessible for all

Break language barriers by offering videos in multiple languages. Replace the original audio with translated tracks to reach diverse audiences. This simple step enhances inclusivity and ensures your message resonates across communities, making a bigger impact where it matters most.

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Combine with other FastPix features

Audio replacement becomes much more powerful when paired with other transformations in the FastPix API. Here are the natural extensions developers usually explore next:

• Overlay audio at a specific timeline
  Perfect for adding background music, commentary, or announcements after swapping the main soundtrack. Combine both in one request for a complete audio workflow. See guide.

• Optimize audio loudness
  Run automatic normalization together with replacement so every video in your library has consistent volume levels - no jarring jumps between files.

• Add subtitles (auto or own)
  When dubbing into another language, pair the new audio with matching subtitles. FastPix supports both auto-generated and manual subtitle tracks.

• Stitch multiple videos into one
  Localize or update audio across clips, then stitch them together into a seamless program, lesson, or campaign video.

• Add intro and outro
  For training or OTT workflows, replace narration in the core content, then automatically wrap it with a branded intro and outro for polished delivery.

At FastPix, we simplify video and audio workflows, making it easier for developers to deliver great viewing and listening experiences without the usual headaches.

We hope you find this valuable! If there are other features you'd like to see, let us know.

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Conclusion

When you’re building a video product, features like audio replacement aren’t just backend utilities - they shape the experience your users actually notice. On an OTT platform, it might be the option to switch languages the way you do on Netflix. In an e-learning app, it’s keeping course material current without re-recording every video. In an AI-driven creator tool, it’s giving users the freedom to swap in their own narration or background track with a click.

For developers, the challenge has always been the same: these workflows are hard to scale with editors and scripts. FastPix turns them into simple, programmable building blocks. Replace the soundtrack, add overlays, normalize audio, or chain it with subtitles and stitching - all through the same API. That means you can deliver features your users already expect, without sinking months into custom pipelines.

The way we see it, dubbing shouldn’t be a production problem anymore. It should be a product feature you can ship as easily as a deploy.

Try FastPix free with 25 credits → fastpix.io/signup

If you want to see it in action first, book a walkthrough with our team.

Want to build it yourself? → Explore the Video on Demand guide.