AoA does increase the local flow velocity on top of the wing as mentioned, but recall the speed of sound decreases in thinner air as air molecules are spaced farther apart and are not as fast at transferring the kinetic energy. Speed of sound is greater in higher density flows, like sea-level atmosphere and water. In water the speed of sound is highest, so water going vessels use audible pings because they are quick in the dense medium. In thin air you need something a bit faster and we use radio waves and microwaves. M critical is the speed at which sound goes supersonic as mentioned, and occurs where the airfoil is the most cambered. Designers look at the required AoA for a given aircraft wing in cruise and make sure it is compatible with Mcrit for acceptable drag. If not, there are tricks you can do to raise Mcrit like wing sweep, flat airfoils, washout, etc.