Yeah they took the Docs down yesterday, there were 404s all over the place. I was able to git the latest code and compile it on my Mac, so once I've done some digging to figure out exactly how we can approach this, I'll try to get my hands dirty and help out. I'm the junior C++ guy (I'm actually starting to learn it just so I can help on this project - it seems similar enough to PHP, so the learning curve shouldn't be too bad) - so if you have any pointers feel free to share them.
I think I'll create a "Player development" board so that we can all share thoughts and information about this as we move forward.
I'm looking at some the source I managed to download before the version drop. A few things of note.
VLC is coded in C, not C++. A few modules are in C++ including the gui, but otherwise everything else including the core library is in C.
It looks like the VLC module system is designed so that each module only has one parent and possibly multiple children. This might make things difficult for mixing the audio.
VLC already has modules for audio mixing. The real question is whether they will mix streams from two separate decoders. The big thing I'm trying to figure out right now is how the module's are linked and what actually does the linking.