Solution 1: The volume of the parent unit may be set too low or may be switched off
Increase the volume of the parent unit.
Solution 2: The microphone sensitivity level of the baby unit may be set too low
Increase the microphone sensitivity level with the SENSITIVITY button or the SENSITIVITY option in the menu of the parent unit.
Solution 3: The baby unit and the parent unit may be out of range of each other
Reduce the distance between the units.