In fact my problem is not so strange: all screens produce noise, electromagnetically speaking, and you can listen to it with a simple radioIt's because of induced currents and magnetic fields.
That carries an extra problem. We have:
* All monitors emanate electromagnetic radiation which depends on the shown image, and they can be captured with a radio.
* Some speakers are capable of syntonizing radio
* My LCD monitor has integrated speakers and always working, just touching the screen.
* Conclusion: each image I see, I also hear it, and I don't need extra devices to do that.
I have a solution: bring the volume to zero, or connect a headphone so that the sound stops going out the speakers. But anyway, if I you want to listen to music, it's better to display a black background at the screen to reduce the radiations and thus interferences.