A paper popper like the one shown in this YouTube video makes a loud sound when moved quick through the air.The air pressure inside the 2 conical structures in the popper would be higher when this popper is moved fast. But how exactly is this difference in pressure converted to sound? Will having different paper types produce different frequencies?
Pretty much exactly off wikipedia, let me know if you have any further questions.
What happens is that air rushes in a blast wave to fill the vacuum created in the pocket that opens. The sound is described as the crack of a whip, which is an example of breaking the sound barrier, although the causes are different.
So its basically the rushing air into the newly created pocket that makes the noise.
Whilst physLads answer is correct, it is perhaps too succinct to be helpful with those unversed in gas dynamics, so I will attempt to explain what a 'blast wave' is in this case.
"My understanding currently is that the pockets created in the popper actually become high pressure pockets ( like a balloon) when moved very quickly through the air. This high pressure releases a small flap of paper suddenly when the differential pressure ( inside the pocket pressure is higher than what is just outside) is enough to push out the paper flap against frictional resistance."
This much is correct. However, as the flap of paper moves away, the high pressure gas pocket that had built up must rush in after it, pealing away layer by layer in a 'rarefaction' that means that, as the paper is moving it is followed by a moderately low pressure, but crucially very fast moving air. The paper is then brought to a very sudden halt (in the frame of reference of the persons hand), but still has a mass of fast moving air piling in towards it. This creates a 'blast wave', in this case a region of stationary high pressure air between the paper the region of fast moving low pressure air. The blast wave is the discontinuity in velocity and pressure between these two regions of air. As more air piles in across the blast wave the region of high pressure air adjacent to the paper increases in size, and the blast wave moves outwards through the air. This 'blast wave' or 'pressure discontinuity' is the loud pop heard.
Edit: you state in comments that you believe the sound to be produced by the region above the paper popper as the paper accelerates into it. It is true that this will also create a blast wave. However the acceleration of the paper is much slower than its deceleration, thus this wave will be smaller. In addition, the rarefaction caused when the paper halts will reduce the size of it yet further. Recordings of a paper popper being used may contain an ear splitting pop followed or preceeded by a much quieter one, the quieter one will be produced by this method.