I lived in a very 'social' shared house in university. The year I had my finals, I usually went to bed earlier than my other housemates who didn't have exams, and honestly no kind of noise would have alarmed me. I slept through it all. I sometimes used to even wake up when visitors looking for the bathroom stumbled into my room by accident. I groggily gave them directions to find it and went straight back to sleep. Thought nothing of it.
College is a different universe and mindset. I would not be ok with that set-up now but back then it seemed normal and, weird as it sounds, non-threatening and fun in a chaotic kind of way. I would not be remotely surprised if the surviving roommates had zero idea what was going on in the rest of the house, as they were conditioned to comings and goings at all hours, between roommates arriving home and various boyfriends and friends on any given night. Assuming they could hear anything from upstairs, considering what a previous tenant said about how noise travelled poorly in that house. And factor in earbuds, which I did not have, and none of it surprises me that the downstairs roommates seemed to notice nothing or very little during the time of the attacks.