I will bring in one of my macs to test this out. My initial thought is that the server looks for file types that it recognizes, like MP3 and M4A files, as opposed to simply listing everything within the directory. It may simply not think the AIFF files are sharable multimedia files. I will encode some AIFF files and try to duplicate this, but in the interim I'd recommend just re-encoding the files into a format that you already know works reliably.