I work with Go and there is plenty of libraries that can extract a audio track from mp4 container and wrap it into mp3 container. I doubt the situation is any different with any other language. You can also write it yourself from scratch. It's not that hard to read the specs and make a working library in a day or two. I know this because I was recently looking into live video streaming so I learnt quite a bit about codecs, containers, protocols and so on.

Though, obviusly, I am not talking about transcoding one codec to another.