You thought you put mp3's on the cd, but the program that you used to burn them to the cd assumed that you wanted to make a music cd, so it converted the files to .cda. Use any one of the many free cd rippers out there to convert them back to mp3's.
Not entirely a good idea. If they were mp3s, then convered back to cdas, you're likely to get audio artifacts if you re-rip to mp3. You're better off doing clean rips from the originals again.