The audio quality is excellent, thanks to VTech's use of DECT 6.0, so you shouldn't hear any interference, static, or echoing when you listen in on your baby. Since the monitor uses a special frequency to relay the signal from the monitor in your baby's room to the parental unit, everything is encrypted and secure. No one but you will hear your baby.
Baby monitors may have a visible signal as well as repeating the sound. This is often in the form of a set of lights to indicate the noise level, allowing the device to be used when it is inappropriate or impractical for the receiver to play the sound. Other monitors have a vibrating alert on the receiver making it particularly useful for people with hearing difficulties.