Get the bleeding edge
The Soprano sourcecode is hosted in the KDE subversion repository. KDE provides anonymous access to the repository in case you do not have a KDE development account.
With the anonymous access the Soprano sourcecode can be retrieved as follows:
svn checkout svn://anonsvn.kde.org/home/kde/trunk/kdesupport/soprano
Browse sourcecode online
KDE also provides a web interface to the subversion repository which allows online browsing of the Soprano source code.