VTech BM3000 - The Parent Unit Does Not Play Any Sound When There Is Noise from the Baby Unit

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Cleaning your video and audio monitor
Your video and audio monitor has a durable plastic casing that should
retain its luster for many years. Clean it only with a soft cloth slightly
dampened with water or mild soap. Do not use excess water or cleaning
solvents of any kind.
Remember that electrical appliances can cause serious injury if used
when you are wet or standing in water. If the baby units and/or parent
unit should fall into water, DO NOT RETRIEVE IT UNTIL YOU UNPLUG
THE POWER CORDS FROM THE WALL. Then pull the units out by the
unplugged cords.