Rhythmbox needs the MPRIS plugin enabled (use the plugins window to enable the plugin)
Modern Rhythmbox versions (2.90.1 and later) exposes a session dbus entry called "org.gnome.Rhythmbox3" when Rhythmbox is actually running. To see this slightly alter your dbus command:
dbus-send --session --dest=org.freedesktop.DBus --type=method_call --print-reply /org/freedesktop/DBus org.freedesktop.DBus.ListNames
Alternatively just use the mpris interface as explained in this excellent Q&A.
If you use the application d-feet then this clearly displays the objects exposed by the mpris interface: