1 / Check the sound level of the monitor
The monitor controls its own sound level. If there's a decrease in sound, make sure that the sound volume hasn't been changed.
From the monitor's screen, click on the gear icon,
Then go to general settings,
Click on Audio Volume
and press the "+" to increase. When the sound is at maximum, the bar is completely green.
2 / Ensure that the call station microphone is unobstructed
The call station microphone is exposed to the weather and external annoyances. It is important to make sure it's not obstructed. For example, in some windy areas, dust can accumulate in the opening, or even small insects may seek refuge there.
To remove any deposit in the microphone opening, blow into it or remove it (with the help of a fine and pointed pick) carefully.