fce and fcp are both retarded when it comes to inserting audio. however, what you can do it put the song into either iMovie HD '06, or iMovie '08, and then save that file. what you do next is, you have to import the file to fcp. then you should have the audio working. on imovie 08 i know that you can just export the file you're working on straight to fc, but i'm not sure with imovie 06. if you have a recent mac, you can use imovie 08, and if you don't have it, then you'll have imovie hd 06, and if you (for some bizarre reason, don't have imovie hd 06 either) then you can download it for free from apple.com.

basically, you put your itunes song into one of those programs, and get that file into fc, and it will work. good luck, and i hope this works/helps.