It's important to know what files can be used by your MP3 player when it comes down to audio quality. Lossless audio files are the best. This includes file types like FLAC and ALAC. These types of files compress audio but do not necessarily remove any quality. Both MP3 and AAC files found in iTunes songs are referred to as 'lossy audio files'. They are compressed to a smaller size than FLAC and ALAC but they remove some sound quality.