Audio CDs have their data in a specified format that's the same for every track. They don't understand mp3s (or WAVs for that matter). In order to test your mp3 files on an audio CD, they need to be uncompressed. But don't worry. Once you uncompress an mp3, you don't regain the data that was lost during the compression. You're just turning that new "worse" audio into a copy that a normal CD player can understand.