Aac, M4a In Itunes


I'm running Tiger on an iMac G5. When I download audio files with the .aac extension, the only application that will play them is Real Player.

My iTunes plays files with the .m4a extension beautifully, but it doesn't recognize the .aac extension. I find this puzzling. When I select iTunes as the default app on a file with an .aac extension, the file loses its Real Player icon but doesn't assume an iTunes icon--it just goes blank. When I change the file extenstion to .m4a, the iTunes icon appears, but iTunes won't play the file.

Repairing permissions and deleting the iTunes preferences file have made no difference.

Is there a simple explanation that I'm missing?

Thanks in advance,