If they are .cda files they are not MP3 files. They are compact disk audio files and should be converted to MP3. They would also be quite a bit smaller if converted I would imagine.
Here is a site that lists all the audio file extensions and what they mean.
http://www.globalmusicresource.com/basics/basics2.html
You should be able to convert them to actual MP3 files with MusicMatch Jukebox or some other audio tool.