if you describe how he looks or link to a picture of one from Google, we can ID him and then you can get a sound file of the actual call for this kind of owl. i can tell you that if you play the right call, he will react.
I've only seen him at night. He looks like a big black, owl shaped shadow. I've never heard him. Maybe I'll check http://www.whatbird.com/ for owl sounds, record them, and play all of them for him.