You can be pretty certain that any piece of music written or recorded in the twentieth and twenty first centuries is copyrighted. There is no way to avoid attaching a copyright to anything newly created once it is set in a permanent form. You can license the copyright in such a way that anyone is free to use it (see the Creative Commons for some ways to do that), or you can publish a statement that you want it to be in the public domain (may not be legally binding). What you need to do is to find out who owns the copyright and get a license to do what you want to do.