libsndfile is being developed by a small but growing community of users and hackers led by Erik de Castro Lopo. People interested in helping should join the libsndfile-devel mailing list where most of the discussion about new features takes place.
libsndfile is being developed using the Bzr revision control system which is especially well suited to distributed development. It is even available for windows, but I (Erik) have not personally used it on windows.
The main archive archive can be found at (yes, the directory looks empty, but it isn't): http://www.mega-nerd.com/Bzr/libsndfile-dev/
Assuming that you have Bzr installed (on Debian GNU/Linux "apt-get install bzr") you can do:
bzr get http://www.mega-nerd.com/Bzr/libsndfile-dev/
to retreive a copy of whats currently in the public archive. To configure, build and verify the build you can do:
./autogen.sh make make check
During the configuration stage, some later versions of aclocal (I see this with 1.9.X) may spew out a bunch of warning messages about underquoted defintions in AC_WHATEVER, but these can safely be ignored.
In order to build the sources from the Bzr archive, you will need the Python interpreter and GNU Autogen which is available using apt-get on Debian/Ubuntu systems or as source code available here