I saw an article by Alyssa Danigelis on Discovery News about one-way sound walls. The article is more about offices, but it seems to me it could do wonders for retail spaces.
Imagine a room where a band is playing. Neighbors can’t hear the music, but if someone outside the room is talking, the musicians can hear it. The concept — a kind of one-way mirror for sound — seems imaginary, but two Italian scientists recently pushed this kind of sound manipulating technology closer to reality.