we built ConvertPrivately as an alternative to cloud-based conversion tools. this is a webapp that uses FFFmpeg and your own CPU to convert video -> audio and audio -> audio. no downloading software, no sending to cloud. entirely on your browser. also it's free, has no ads, and open source. you can see the code and improve it. you can even host the website yourself.

https://github.com/convertprivately/convertprivately/