Why can't I save AAC files to music folder?

My itunes import settings indicate that recorded songs are saved as AAC files. But in My Documents in the music folder, where my Bose soundtouch wifi app looks for them, the files appear as MPEG-4 format, which the bose app can't read. It can read AAC files. How can I get itunes to save the songs in the AAC version? (or WMA or MP3, all of which should work)