There are all kinds of licensing protections available to musicians. The main purpose of organizations like ASCAP is to make sure that artists are compensated for their work. In the past this has meant that the unknown artist just puts their music out there without any protections at all.
But now there is Creative Commons Licensing. It is a way for musicians to determine what level of ownership they want to maintain while giving away songs. You can read about it here: http://creativecommons.org/
The idea is that you choose what level of protection you want, create the legal document licensing your songs, and then you are protected. You can say if people can sample your songs; re-record them; and if they can sell derivatives of your songs. A breakdown of the licensing options available can be found here: http://creativecommons.org/about/licenses