Don't copyrights expire after 20 years or something. I know that songs are used in commercials all the time and after some period of time the copyright on the song expires and they move into the public domain. At that point the song can be used by anyone without paying for it's use.
No, copyright lasts a lot longer. 70 years after death of author, or if work of corporate authorship, the shorter of 95 years from publication, or 120 years from creation. That's why it takes so long for something to become public domain. When a song is used in a commercial, it's because the copyright holder granted permission.