Logic Pro/Express 7.2 now allows you to play compressed audio formats such as AAC, Apple Lossless, and MP3 directly in the Arrange window. Because these files are smaller in terms of disk space, you might think that they would also require less CPU and disk drive bandwidth to play back. However, this is not the case.
In fact, directly playing compressed audio in your song may actually require more system resources than playing back an equivalent number of uncompressed audio tracks. In situations where the added overhead becomes a problem, you may get better performance by converting your compressed audio tracks to AIFF or WAVE files.
Select the relevant regions in the Arrange window and choose Arrange > Audio > Convert Regions to New Audio Files. A file selector will appear, allowing you to choose a destination file format for the copied files.