How fast the gas expands through an opening depends on the pressure ratio between entrance and exit, and for ordinary air the flow reaches the speed of sound at the narrowest part when the pressure ratio exceeds about a factor of two. The flow never becomes supersonic in the constriction itself, but after the constriction the flow can continue to accelerate to supersonic speeds, especially if the constriction is followed by an expanding nozzle (converging-diverging, or de Laval nozzle), although this is not strictly necessary. For example, one can observe shock diamonds in the air coming out of an ordinary plastic bottle filled with slightly compressed gas: https://www.youtube.com/watch?v=61tsXGUc6nU
I wonder if the shock waves visible in the video are still due to the converging-diverging aspect, just with no bell around the diverging part.
In rockets, there is a so called "aerospike" engine [1], which does not have any bell-shaped nozzle at all, but instead uses the ambient air to always shape the flow for an ideal expansion ratio. In theory it is great, but in practice it remained mostly a pie in the sky.