A monitor is a dumb screen that has no internal electronics that can process either video or audio. All the processing is done by the PC based graphics processor.
Most important, a monitor does not have a receiver so you cannot see any TV signals on it. If you want to do that you have to buy a graphics card that has a TV receiver built in, and route your set top box and satellite TV box through your computer.
In addition, the speaker in a monitor will usually be of very low power (generally 5 watts or so). It is not meant to be used to listen to music or the sound that accompanies a TV program. You are expected to sit some 2 to 3 feet from the monitor, or use headphones to listen to sound from a PC. That is the reason for low power.
In other words, a monitor will not work without a computer. That is the reason TVs are more expensive, as they have the electronics to work independent of any external processor for both video and sound. They also have an internal amplifier and speaker that can handle the sound that comes with video (such as TV programs or a movie)
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