I find the sounds associated with people having sex to be a big turn on for me. The first time this I experienced this was as a young old dude living in my first apartment. I had fallen asleep when I was awakened by the sounds of the neighbors in the apartment next door going at it furiously. The walls were paper thin, so I could clearly hear parts of their conversation as well. She was a very vocal and nasty (in the good sense) woman. At first I tried to ignore it and go back to sleep, but in short order the voyeuristic aspect of eavesdropping on their sexual activity kicked in and I became aroused as hell. As I recall the 3 of us came at the same time.