Apple’s tech support article describes it the best. FCP cannot playback an MP3 file in real-time due to the compression. As a newbie this seems strange since the DV codec requires decompression as well but I guess the assumption is that MP3 files aren’t appropriate for video projects so they didn’t optimize it for RT.
Since I’m working on something “straight-to-YouTube” I really don’t care that my soundtrack is of the highest quality but rendering audio is no fun either. Rather than re-rip the music from CD media I’m converting the MP3 to AIFF as described in the tech article using QuickTime Pro. The result was surprising: a 6.9 MB file in MP3 format was 133 MB in AIFF format.
On a whim I created a batch job in Compressor 3 and the same file took mere seconds to convert and was half the size: 61 MB. I expect to be using QT Player less and less.