Ask HN: What audio/sound-related OSS projects can I contribute to?

Hey! I wanted to research the possibilities of participating in open-source projects, preferably related to audio or sound processing. Do you have any projects to recommend? Preferably ones that are either technically interesting, solve a useful problem or have lively community and are in active development.

I'm not specifying anything more (language, etc.), so that I can get as wide of a selection, as possible and so that this post is useful for others more.