Great Horned Owl Vocalizations
Most people know that Great Horned Owls hoot. But most don't realize that there are several types of hoots, plus a variety of chitters, squawks, hisses, and bill clacking also in their repertoire. The International Owl Center is working to record the complete vocal repertoire of the Great Horned Owl along with associated behavioral context, which is why they is breeding a non-releasable pair of owls in captivity.
Vocalizations in owls are hard-wired, not learned, so their vocalizations in captivity shouldn't differ from owls in the wild.
So far these are the types of vocalizations that have been documented:
Emphatic, sometimes with single squawks
Hoot without the last note
Hoot with an additional note at the end