Rhythmbox is maintained primarily in GNOME SVN. You can also see the current code in the SVN Web service.
If you don't have a GNOME SVN account, you can check out the current developent tree (trunk) using
"svn co http://svn.gnome.org/svn/rhythmbox/trunk rhythmbox".
If you do have a GNOME SVN account, use
"svn co svn+ssh://yourusername@svn.gnome.org/svn/rhythmbox/trunk rhythmbox"
The Rhythmbox developers, and others interested in Rhythmbox, hang out in thr #rhythmbox IRC channel on irc.gimp.net. Feel free to stop by and ask questions or discuss some ideas.