2.4GHz Wireless Technology: This baby monitor gives you great peace of mind when your little ones are asleep upstairs and you are downstairs Night Vision Infrared: Perfect for anytime use, enable you to keep eye on your baby all night in your bedroom, no need to go to the baby's room 2-Way Communication: If baby seems a little agitated you can speak some soothing words and put your baby at ease Temperature Monitoring: You can check what the temp is in the babies bedroom, which helps you to ensure your baby stay in comfort and safe environment Lullaby Function: You can also use your video baby monitors to play soothing baby lullabies which are built into the baby monitors If you can't hear the sound, please firstly check if you turn on the VOX mode The baby unit doesn't have built-in battery, it need to be plugged in at all times The parent unit has a built-in battery (500mAh) which lasts around 2 hour (the actual using time largely depends on how often you use it), the battery will last longer if it works under VOX mode Working Range: 100m. There must be without obstructions which would influence the signal