There is a new Plugins module on the CVS. The plan is to gradually open-source all our plugins by placing them in this module. New plugins, including XAudio2, will be developed on this module as well, and the preliminary (but not yet useful) code for XAudio2 is now on there if you're feeling curious.
This will provide plugin developers with an excellent resource for example code, all kept up to date with new releases. Hopefully in future it will cover a broad range of plugins. Additionally, our plugins are currently managed by different code management software which isn't as ideal as CVS, so it makes sense to have all the code hosted in the same place.
Being an open-source project, the open-sourcing of our plugins is long overdue. We haven't open-sourced existing plugins because many of them have been in development for one or two years, dating back to very early (and frankly ugly) versions of the SDK. It would be counterproductive to share messy code & projects (even if they work smoothly) because ideally other developers should not be writing their plugins like that! Also, it has taken longer since the SDK was known to be changing. However, the SDK has been stable for some time now so can be assumed safe to develop with, with no breaking changes planned. Over time, we plan to convert our plugins to the latest SDK version, and put them up on the Plugins CVS module.