Re: how come it is easy to get ultrasonic air flow under vacuum ?

I'm not actually able to reproduce your numbers. We take atmospheric density, which is 1.2 kg/m³, and want to have a velocity jump of 100 m/s, or about Mach 0.3. If V₂ >> V₁, then the pressure jump would be 6,000 Pa. That's about a factor of 1000 smaller than your answer, and it would be exactly 1000 had I used an atmospheric density of 1300, as you did.

Actual atmospheric pressure is, as you say, almost exactly 10⁵Pa. Assuming that you had vacuum on the other side, then, you could get a velocity of 1700 m/s, or about Mach 5.