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 (SCD520, 525, 530 and 535) in the parent unit menu.

Solution 3: The baby unit and the parent unit may be out of range of each other. Reduce the distance between the units.