Get the bleeding edge
The Soprano sourcecode is hosted in the KDE git system. 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:
git clone git://anongit.kde.org/soprano.git
Browse sourcecode online
KDE also provides a web interface to the git repository which allows online browsing of the Soprano source code.