First of all, what do you mean with "CDA"?
If with CDA you mean CD Audio, then FreeRIP can convert them to MP3, Ogg vorbis, WMA, WAV or FLAC. FreeRIP is a CD Ripper, a software that can extract audio tracks from audio CDs and encode them in various formats. Download FreeRIP MP3 here.
If with CDA you mean files with the ".cda" extension, than it is likely that you are in troubles because those files don't actually contain any sound information! Please read here for more information about .cda files.