To amplify the signal, you can increase the sensitivity of the Analog channel that the microphone is plugged into.
To do so, double click on the sound sensor icon. A calibration window for the Sound Sensor will appear. In the Calibration window, you will see a setting for "Sensitivity" that is normally set to Low (1x). You can click on the arrow next to the setting and, depending on which interface you are using, change it to Med (10x) or to High (100x). Change it to the largest setting available. Click on OK to accept the change.