You are correct that "music" is not copyrighted any longer. However, you probably used a specific recording of that music, which is more then likely still under copyright. Unless you recorded the music yourself, or have permision from the person who recorded it, then you should not have used it.
See
http://www.pdinfo.com/identify.htm and
http://www.pdinfo.com/record.htm for more info.
It sounds like you dodged a bullet. Just remember next year.