Sonic is a 100% client-side audio and video converter. Every file you process stays on your device — always. Nothing is ever uploaded to any server.
Sonic is a free browser-based tool that converts audio and video files locally using FFmpeg compiled to WebAssembly (WASM). It supports virtually every common media format — MP4, MKV, MOV, AVI, WebM, MP3, WAV, FLAC, AAC, OGG, OPUS, M4A — and runs without any server involvement whatsoever.
Most online converters upload your file to a remote server, process it there, and then let you download the result. This means your video or audio — which may be private, confidential, or simply personal — travels across the internet and sits on someone else's hardware, even briefly.
Sonic eliminates that entirely. FFmpeg runs directly inside your browser tab via WebAssembly. Your file is read into browser memory, processed locally, and the output is saved to your downloads folder. At no point does any data leave your machine.
Sonic loads FFmpeg as a WebAssembly module (approximately 30 MB, cached by your browser after the first visit). When you initiate a conversion, the file is passed to a Web Worker — a background JavaScript thread — so the user interface stays fully responsive during processing. Progress is reported in real time via a message channel between the worker and the main thread.
Sonic is part of RuntimeHub, a collection of free, privacy-first browser tools built by RUNTIMEZERO. Other tools in the ecosystem include Squeezy (image compression), HeyC! (HEIC conversion), and more.
The project follows an anti-SaaS philosophy: no accounts, no subscriptions, no data collection, no artificial limits. Tools are supported by non-intrusive advertising so they can remain free for everyone.
Yes, completely. No account required, no watermarks, no conversion limits, no upsells. Sonic will remain free as long as it exists.
For questions, feedback, or bug reports, reach out via RuntimeHub or find us on Instagram at @run.time.0.