Hi, its pretty measly less than 4 Gig as I recall, just my own feeling, I prefer to use a Mini USB stick so that the built in memory is not worn out by playing music. ( We have the base model L in canada FE for U.S. )
The software on our car is a Canadian version and I suspect is missing features found in higher trim model, which I was not aware of any difference when I researched before buying.
I also tried to reply yesterday but the post disapeared. Anyway the Base trim model Entertainment unit does play Flac files much to my surprise, in the manual it indicated no MP3's above 320 bitrate.
I also test drove the Top trim touring Model and the Harmon Kharmon sound system was very impressive, however i didn't try any USB music.