Hi
I'm using sonar 8 and also learning to use sonar x3, but it both versions of the soft I get the error when trying to import some aiff files, not all of them, but some. I try to import from different loop libraries and for some unknown reason it will import from some libraries and from some it won't. 
I've no idea why this is happening. I read that Sonar won't import "read only" files, so I uncheked that from the folder options, and also files with metadata imbedded in them, but I don't know how I can tell if these aiff files have any other data imbedded.
 
Has anyone come across this problem? It really bugs me to convert each and every loop in an external editor

I had this problem importing mp3's. The fix was to convert the files to wav before I import them. Something about the metadata that's stored in mp3's. Try converting them to wav and see if that helps.

The AIFF format is a general-purpose wrapper that doesn't necessarily even have to contain plain ol' audio. It can contain up to 6 channels of PCM, compressed audio (e.g. iTunes), loops, instrument definitions, DAW-specific settings, metadata or even MIDI data. Or any combination of the aforementioned data types in one file! Here's the full spec.
 
I'm guessing here, but my assumption would be that SONAR can only import basic stereo PCM and loop AIFF files. It is possible to see what's in the file using a hex editor and knowledge of the file structure (everything you need to know is in the spec linked above), although I wouldn't be surprised if someone somewhere hasn't written a program to spell it out for you.
 
But the easiest solution is to simply convert the files to WAV. Tools do exist that can perform batch conversions, but I've never had the need to go find them. I'm sure google can. iTunes can convert them, but I don't know if it can do batches, as I will not allow iTunes anywhere near my computer.