I have heard several roommates, floormates, etc. Mainly due to thin, poorly insulated walls.

I had one roommate, with a ridiculously hot gf, who got upset because I had never heard them having sex. His gf took it personally, so she went out of her way to make noise and act rowdy in bed so that I could hear her. Everytime after that I still told her that I could not hear anything just to see how far she would take it. Knowing that she was thinking about me and putting on a show for me was pretty fucking hot.