My cat comes running into the room whenever I turn on the tv or stereo and sits right next to it. If I turn it off, he meows over and over especially when I turn off the music. He also lays on the DVD player, stereo receiver and tries to get on the tv. Will that harm him?
No, it won't harm him, though it may shorten the life of the electronics.
He likes lying on these devices because they're warm.
The risk is that by doing so he may block ventilation that they need to keep their components within their intended operating temperature range, or fur may accumulate inside them with the same effect.
You'll need to decide whether the risk of breakdowns occurring sooner is an acceptable cost for keeping your cat happy, or whether you want to declare these off limits and let him find a warm spot somewhere else.
He also may come running into the room because these noises indicate that the human will be there for a while and he likes being near you. My cats definitely respond to the noise of my turning on the living room lights that way; it usually means I'm settling in to read for an hour.
(Though it's also true that some cats do seem to understand music at least enough to like some kinds. They probably can't play the prediction game which makes music so much fun for us, but maybe they can at least hear basic chords or rhythms. I'm sure someone has tried to do a study on that.)
Re "electronics radiation": @Rotem, what radiation are you talking about? Most of what most electronics emits as electromagnetic radiation is infrared -- heat -- plus a bit of 60-cycle hum (which we're all immersed in anyway). Electronics equipment is shielded to prevent emitting (or, more importantly, picking up) RF, and the amount it would emit would be less than a toy walkie-talkie even if unshielded... far less than a cell phone.
If you have a specific concern, we can address it more precisely. But electronics is not usually a hazard, unless you start talking about microwave ovens or high voltage. Really.