What's going on with the first couple at the beggining??

The professor and his student were the only neighbors within the vicinity of the family's mansion so that included them in on the bodycount of people who had to die since they were there at the wrong time. I agree that leaving the bodies on display and the music blasting didn't make much sense to the grand scheme, given that the masked murderers were not killing for sport but rather money.

I chalk this up to the fact that the movie was intended to be a black comedy, so certain elements of it are meant to taken or thought about too seriously. It's probably up there with the "Fuck me next to your dead mom" scene or the blender death.