It could be because of the DRM protection. Or maybe your import settings on your Playstation.
1st, there's alot of programs out on the web for removing DRM. 2nd, I personally use these settings; MP3 320kbps.
HOWEVER, I guess the answer would be the protection.
I've tried this a couple times - both were unsuccessful. Although GT5 does accept .m4a/.aac files, it won't take them and just throw up a "this music type is not supported" error.
Has anyone had success at making it work?
I've bought tracks on itunes and they work on GT5, you just have to go to the "Make mp3 version" option in itunes and transfer that file to your PS3.