T-Rex: Okay, so let's assume you know the position of every air molecule in the room.

T-Rex: Things are going pretty well for you, my friend!

T-Rex: Someone in the room says something to another person, and then they both leave. Assuming you know exactly how they moved when they left (and therefore how they displaced the air!) then any remaining displacement is due to sound waves. By looking at the location of the air molecules in the empty room, you can reconstruct what was said in the past! Dudes! It's a perfect, UNDETECTABLE listening device!

Utahraptor: But how much particle displacement happens at conversational levels?
T-Rex: LOTS, probably!

Utahraptor: I dunno. Sound's a wave that travels THROUGH the medium it's in, right, so I'd imagine that's mostly oscillation, not displacement. If you talk to me from across the room, I'm not being splattered with air from inside your lungs.
T-Rex: Oh REALLY?

Narrator: SOON:
Off panel: T-Rex
Off panel: It doesn't count if you're just spitting on me