Common Music
Common Music (CM) is an object-oriented Music composition environment. It produces sound by transforming a high-level representation of musical structure into a variety of control protocols for sound synthesis and display. It runs on GCL, CLISP, CMUCL, OPENMCL as well as some other Lisps.

There was some question previously about the license for this package. From the project page: "CM is released under the LLGPL (Lisp Lesser General Public License)."

For more information, see the CM web page on SourceForge or the CM mailing list.

A common-music page exists to make this ASDF-Installable.

There is a new e-book out by Heinrich Konrad Taube (creator of CM), called Notes from the Metalevel An Introduction to Computer Composition, about computer music representation and composition using CM (and also includes quite a bit of introductory Lisp).