So Sonic Foundry sold the as a separate add-on. You’d install it, and suddenly Sound Forge, Vegas, or ACID could export pristine (for the time) MP3s — fully customizable bitrates, VBR, even stereo modes.

Originally developed by Sonic Foundry before the brand was acquired by Sony (and later Magix), this plugin was essential for high-quality MP3 encoding during the early 2000s. Sonic Foundry MP3 Plugin 2.0 - magix.info

The industry standard open-source encoder, often used via Audacity. Magix Sound Forge Pro: