I don't think anybody has looked at the audio format yet either, but that's a bit lower on the list of priorities.
Some of it is fairly obvious, some of it less so. Then there's a bit of research to do figuring out which files need to be edited in order to change how the game functions and loads stuff. The next step really is getting the game to load totally new models, which as Syd mentioned shouldn't be too far away now unless there are any more unexpected pitfalls. Currently, I'm working on reading + rebuilding the model format, and it's actually possible to get the game to load edited models at the moment. As far as I'm aware, SydMontague and myself are the only people working on building modding tools for the game, which are going to be necessary in order to create mods.