Actually, in the process of compression, more molecules are squeezed into a smaller volume. This DECREASES velocity, internally within the rotor/stator arrangements. To compensate for this, the external ducts (compressor cases) are converging to reverse this effect and keep velocity up!
And exactly the reverse happens in the turbine stages.
If you can find a rotor/stator airflow velocity vector diagram, showing both high-power and low-power ops, you can begin to appreciate the complexity.