Diagnosis
This model has a manual vent damper that you physically set during installation. If it's not venting outside, the damper is likely still in the 'recirculate' position, or the external ductwork is blocked or disconnected.
How to Fix It
First, check the damper position inside the microwave. 1) Remove the grease filters. 2) Look up into the vent opening. You should see a metal damper (a flap). If it's horizontal (closed), that's recirculate mode. If it's vertical (open), that's outside venting. 3) To change it, you'll need to reach in (maybe with pliers) and carefully rotate the damper shaft 90 degrees. It might be stiff. You want it open for outside venting.
If That Didn't Work
If the damper is open but you still get no airflow outside, the problem is in your ductwork. Check the exterior wall or roof cap for a stuck or frozen damper, or a bird's nest blockage. Also, ensure the flexible duct from the microwave to the wall hasn't collapsed or come disconnected—this often happens if it was poorly installed.
Heads Up
The fan speed will sound different between recirculate and vented modes because it's working against different air pressures. If you switch to outside venting, make sure the fan isn't just running with the damper closed, as that strains the motor.