Final Cut Studio 2 supports the preview and playback of CAF (Apple Core Audio Format) audio content.
CAF content included with Final Cut Studio 2 is compressed using the Apple Lossless codec. When you use this CAF content in Final Cut Pro, you can preview it normally in the Viewer, but it will need to be rendered in order for it to be played back in a sequence at full quality.
Note: Final Cut Pro does not support compressed audio formats for Timeline playback. This includes "lossy" formats such as AAC and .mp3, as well as the high-quality Apple Lossless codec used to encode the Final Cut Studio 2 content for Soundtrack Pro. For Timeline playback, simply render the audio first.