For others who wonder the same: the announcement is from the 19th this month and they licensed it under Apache 2.0.
This is really cool to see! I hope they are testing the waters for releasing the source engine as free software. I remember Gabe Newell stating that they would consider it if people were interested. Doesn’t have to mean anything, but a woman can dream~
I don’t get it. Is it just audio tweaking tools that developers can use, that they have now made open source?
It’s more than just a simple tool with which you can tweak audio. Some features are listed at https://valvesoftware.github.io/steam-audio/. The tool is mainly intended for the development of games (also under Linux).
And yes, it’s basically just about the fact that Steam Audio is now completely open source.