Because there was some fiasco with licensing with the newer Xfree86, so all future Debian installs will use Xorg. Since you're running testing, that's the way everything is going to migrate.
Maybe the old xfree86 doesn't have the right system calls for the new xmms... Are you running a mixed stable/testing system? Because from the testing package
repos, it looks like the xserver-xfree86 package is just a dummy package to xserver-xorg.