Communication in langauge and music can be discussed in terms of the signifier and the signified. Recall our earlier example of a message consisting of the word, "Tree." This word is delivered to the recipient who then interprets (I use the word "decode" or "unpack" to mean the same thing) it in various ways.
If we get a little more precise with our terms, we have two closely related entities, the signifier and the signified. In our example, then, we have:
The word "Tree" = The signifier
The image of a tree = The signified
Another word for "signifier" is "sign." What defines a sign/signifier? Generally speaking, anything can be a signifier. The most obvious examples are encountered every day: spoken words, road signs, dashboard "check engine" lights, pirate flags, McDonald's golden arches, and so on. The burden of decoding is placed on the recipient of the message in this system. Remember, though, there is no guarantee any particular signifier will be interpreted a certain way. We are trained through the transmission of culture to understand what red lights, golden arches, and warning lights mean.
What are musical signifiers? Just about anything--notes, chords, scales, musical gestures, electronic effects, the posture and/or attire of the performers, the orchestra's crescendo, etc. Each element of a musical performance could potentially communicate something about the piece being performed (or the group performing it) and therefore each element could be considered to be a signifier. The recipients of the musical message (the audience) act as interpreters of these elements, constructing their own meaning.
Since the general consensus is that since absolute music generally does not (or can not) refer to anything specifically, the "one signifier, many possible signifieds" maxim is usually applied. (There are exceptions in certain types of program music, musique concrete, and so on)
Because of the slippery nature of "meaning in music," a common approach is to explore ways in which musical signifiers correlate to certain elements in society as a whole. Certainly, there is room for multiple interpretations (as the musical gesture "X" does not necessary mean "Y" in all cultures, in all times, in all situations, or even to individuals within a culture), but that does not mean one cannot--or should not--investigate how music reflects or fits into a society. In fact, that investigation (according to ethnomuiscologists) is far more meaningful than any attempt at analyzing musical ideas separately from their cultural context.
Perhaps some examples might be useful. Here are some crude attempts at "signified/signifier" correlations that might be worth discussing in 21st century U.S.A. These are only examples to show how musical signs can correlate to certain segments in society. I'm choosing just a few characteristics (emotional, socioeconomic, etc) and attempting to show how one might show a relationship between musical elements (signifiers) and cultural dimensions (signified).
Signifiers: A piece with a slow tempo, a wispy female solo voice, synthesized strings, and gently arpeggiated piano chords Signified: Vulnerability, love, romance, social acceptance
Signifiers: Large organ, Bach prelude and fugue, large, spacious and resonant church Signified: Mainline Christian church, tradition, establishment
Signifiers: The Beach Boys Signified: Baby boomers
Signifiers: High-pitched male voices, electronic/synthesized accompaniments, drum machines Signified: Dance clubs, night life, GLBT lifestyle
Signifiers: "Heavy Metal" Rock band with long hair, double-bass drummer, shredding guitar solos Signified: Lower-class white America, drug use, nihlism
Now, I emphasize these are humble examples, and because of the fluid nature of this kind of exercise, It's possible you disagree with the depth and/or characterizations I'm listing. Each example finds resonance in certain sub-disciplines in Musicology (fields like music therapy,psychoacoustics, queer studies in musicology, and marxist cultural critiques come to mind), and I'm summarizing these broad areas rather crudely.
With that caveat in mind, though, I think you can agree that there are generalizations we can make about the relationship of music to our culture.
In modern theory, the most important link in the communication process is the recipient of the message, as they are responsible for constructing meaning out of the message and interpreting it. We typically split the message into two parts--the signifier (or sign), and the signified. In our example, the word "tree" is the signifier, and the signified is whatever the listener chooses to imagine a "tree" being. Because of the possibility of multiple interpretations, there is an imprecision to spoken language that we all accept.
Musical communication introduces additional abstractions that seem to make the conversation more difficult. We can agree that there are multiple elements in music that may be construed as being "signifiers," but we can run into great difficulty when we try to answer the question, what is meant by this?
Modern musical critics tend to relate these various musical signifiers to the societies that value them, and thus the disipline tilts toward discussions of "how music fits into culture." I provided some brief (if shallow) examples of how one might interpret different musical signifiers, relating them to different facets of society.