If you can buy this music anywhere, then they would likely be copyrighted. There's still a lot of music available for download through questionable sources. So, I'm guessing that you purchased these somewhere; or, these are true Public Domain, not songs pulled from any commercially available source, like a CD that could be sold to the public?
What error message do you get when you try playing one of the files in iTunes? Have you tried importing the songs into iTunes into a different format (AAC, Apple lossless, etc.)? Have you tried playing the files through your Quicktime Player? If that works, you may be able to export the file to a different format that will work with iTunes. Or, you can try the same trick with another app that will play mp3 files, and could export files into other usable formats. There's lots of ways to 'skin a cat (mp3)'
