CL-XMMS provides UFFI-based Common Lisp bindings to libxmms, the library used to control a running instance of XMMS. You can add files to the playlist, play, pause, adjust volume, ... pretty much everything that can be done through the interactive XMMS interface.
No releases yet, but the code in the CVS is quite usable.
You can browse the CVS repository or download the current development tree via anonymous cvs, as described here