‘There is no such thing as zero volume’ - can you expand on that?

Surely constant unchanging air pressure has zero volume?

Volume is the range of variation in air pressure, right? That can surely go down to zero.

It can also only meaningfully go up to about double the absolute mean air pressure, before what you are talking about becomes shockwaves of overpressure.

Air pressure is a statistical measure; given a room with zero net variation in pressure, I can always find a volume in that room with positive pressure, down to measuring Brownian motion of atoms. Now try to design a measurement apparatus that can only sample a small volume to measure variations in pressure, and you can understand why (sound) volume can never go to zero.