Actually they use music from numerous sources, including operatic, classical, pop, rock, hip hop and others along with original scores. It mostly depends on their budget and the mood they are trying to set.
If the trailer is any indication then it doesn't even have opera music. Opera music tells a story. The music in the trailer is a classical style with angelic like singing in it. Its neither classical or opera. Just because you hear string instruments and some singing you can't understand doesn't mean its classical or opera. All you hear is string instruments playing ominous music.
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It mostly depends on their budget and the mood they are trying to set.
I would think most games would pick music tunes that do not have a copyright to contend with.
A.. -.-. . -. --.
Does it have both classical and opera (as well as other genres)?... Yes.
So tell him he's wrong and you get partial credit. :-)