Technically there is no reason why security cameras can’t record sound just like any other type of video camera or the camera on your phone. The technology is the same and is readily available. The reason that many security cameras either omit this feature or operate with it disabled is that there are often legal prohibitions against using a security camera with sound recording turned on.
Why you may not want to record audio
Surveillance, especially audio surveillance, of people who do not explicitly agree to it is not always legal. Video is usually acceptable in public spaces, which can include the exterior of your home and retail space. Audio is significantly more protected than video, though this varies from state to state. In California, it is generally not permitted to record audio with a home security or commercial audio video system without the express permission of the person(s) being recorded. In the work environment there may be further limitations. For example, some unions prohibit employers from monitoring their members with audio or video.
Practical sound recording limitations
Irrespective of the legal ramifications, there are practical reasons why good quality audio is hard to achieve with a commercial surveillance camera setup or home security installation. A camera that does have sound recording ability may not give you very good audio simply because it is difficult to record good sound under the conditions that prevail. High quality audio such as what you hear on TV shows is obtained with the microphone held close using a boom pole or other means, which is not possible with a microphone on a security camera that is high up and out of the way.
What this means when buying security cameras
Security cameras are a great idea to protect your home or business, and provide useful information if something happens as well as acting as a deterrent. You may wish to focus on video quality rather than the audio specifications if you know you will not be using audio. If the camera is being used outdoors at night, you will want one that performs well in low-light conditions. If video is being recorded, assuming that is allowed, you will want to know how much it can store, and whether it can be live-streamed.