Is my audio uploaded?

No. Files are processed on your device, inside your browser. Nothing is sent to a server.

Which browsers does it work in?

Current Chrome, Edge, Firefox and Safari. Files are decoded and encoded with the codecs built into your browser, so what it can read depends on the browser; M4A (AAC) output needs Chrome, Edge or Safari. MP3, WAV, FLAC and Opus work everywhere. The MP3 and FLAC encoders are downloaded from this site the first time they are needed.

Normalize Audio - Free Online Tool

JavaScript is turned off. InspireTools processes files in your browser, so the tools need JavaScript to run.

Even out loudness so every track plays at the same level.