In signal processing, there might be a lot irrelevant or unintended (by the sender) sounds being received; in submarine detection and tracking, the sounds a submarine makes are only a small part of what one hears. In both cases, the unwanted or irrelevant sounds are called "noise." The rest isn't "music," of course, but data or information—either the voice or morse-code message that one wanted to hear, or the sounds of the submarine that one is trying to track.
Metaphorically, then, when a great deal of information is being received, much of it irrelevant, a major task is to find the relevant information among all the noise. Once the relevant or significant information has been identified, everything else is noise.