The amplifier on the board is wired to give a 2.5 volts signal when there is no sound. When there is some sound the exact shape of the signal will show in voltage variations around 2.5 volts = 512. The variations will be fast....
When you print numbers @9600 baud to the monitor window you are sampling around 7 ms, which means you will hardly notice any signal faster than 20 Herz. The mike might not even hear that...
So e.g. change the bitrate to 110k or so and sing with your deepest voice.. Then you might read something like a slow sine 