Hey,
Anyone here know a good (free) program that lets you convert various audio files (in my case, from MP3 to OGG)?

Audacity, FFMPEG, etc. You'll find ffmpg is the big one that everyone uses at the end of the day.

+1 for ffmpeg
it does pretty much convert anything in anything

Thanks.

Audacity would be the easiest to use don't know about the others though!

I have been using Audacity for as long as I can remember. I never felt the need for anything else.

Pazera Free Audio Extractor - it uses the FFmpeg engine, and besides conversion, it can also extract the audio from video files.

Ditto. I have a script to convert mp4 files to mp3 so i can listen to it on my mp3 player while walking. I recommend FFMpeg

Audacity is getting another vote.

XMedia Recode, which can convert pretty much any media file.

https://xmedia-recode.de/en/index.html

Audacity uses either lame or ffmpeg as a backend. Audacity is audio editing, but it follows "the unix principle," which is you should have a bunch of small programs do tasks and other programs should use those small programs to complete more complex overall tasks.