I wonder how this differs by area and building type. My apartment walls in the Bay Area were razor thin - I couldn't believe how much could be heard. But in the four apartments I've been in Manhattan (all ~50-100 years old), plus a good friend's new high-rise that I frequent at all hours, I've never heard my neighbors.

In fact, in my current apartment I have not once heard my roommate when my door was closed despite the bedrooms sharing a wall.