If you were to do a plane or world where music interacts with
everything and instruments are essentially able to influence reality
based on how they are played, what would be the practical
difficulties? Where might such a world fall apart, and what would you
suggest as caveats to prevent that from happening?
This is an extraordinarily complicated question, but -- with all due respect -- the first clarification or moderation applied to the question has taken it in what I think is a problematic and overly narrow direction. Music is not intrinsically about vibrations, sound, pressure, or anything like that. It is what it is. See Plato's various discussions (often highly negative).
Music affects people, or the world, because it does. Now you have to immediately ask, "why?" That could be because of vibrations or sound or whatever, but it could be because of harmonic relationships. For example, you could say that any absolute relationship of 1:2 or 2:3 has an automatic effect on the world, because in fact, normally speaking, nobody and nothing ever gets close to such precise relations. We always approximate. So we try to build a house that's twice as long as it is wide, but in fact we get kinda close to a 1:2 but not really. Thing is, we can tune an interval very precisely, because if we put a bar on a string at nearly 1:2 (5" from one end, 10" from the other), and vibrate the string, we can roll the bar up and down until our ears tell us that the relationship is getting closer, and then any ear that isn't actually tone-deaf can home in on the correct relation until we get very precisely to 1:2. Or 2:3. 3:4 is trickier, but not by much. And that gives us octaves, fifths, and fourths, and they are perfectly (justly) tuned. And that is not normal: few if any other relations in the real world can be "tuned" so perfectly
So why is that?
Well, maybe it's because the universe is structured around these special relationships, but only music is able to bring them to the fore. Or maybe it's because the Gods like music, and have infused its basic relations with special divine qualities. Or maybe it's because the stars are exactly in those sorts of relational distances and aspects, and this reflects down into the sublunary world through music -- in which case, why music alone?
So far, all this is pre-created: this is Greek music theory 101 through elementary medieval European music theory 101. And I do mean 101: you have no idea just how complicated this stuff could get.
But to answer the question: why does music affect the world? Because the world is built on musical relations, from soup to nuts; more accurately, Creation itself is built on those relations. When we perform or play or think about musical harmony, we are tapping into the structure of the universe itself. Nothing else but pure Euclidean mathematics (which has no real-world analogy) is like this.
So...
What are you trying to explain, exactly, and how does it work in your universe? Because I can explain music having pretty much any implication you want, without having to invent anything whatsoever. You just have to realize that "music" does not mean "sound": it means complex mathematical relations that may be expressed sonically but are weirdly drawn toward perfect integer relations... just to begin, at least.
Background Thinking
Bear in mind that the category "music" is in no sense obvious or automatic. While we know of few if any cultures whose people do not produce something we in the modern West would classify as "music," we also know of few if any cultures that have a category or classification that maps the modern Western "music" at all smoothly. To give the obvious example (which also lines up with my previous remarks, historically), in the Middle Ages, "musica" (Lat.) was classified together with astronomy, geometry, and arithmetic; these are the quadrivium, the topics of study and analysis founded in the nature of the Creation as God chose to make it. Then you have the trivium -- rhetoric, grammar, and logic -- which are the topics founded in human thought and arbitrariness (in the technical sense). The reason "musica" was in the quadrivium and not the trivium was not only that "musica" is rooted in harmonic relations and so forth. It's also because what we usually mean by "music" today was a narrow subset called "musica practica," music as it is actually done in human life. And this was deemed uninteresting, unimportant, and largely insignificant, because what's interesting and important about music is how it is embedded in the Creation, NOT what you happen to hear somebody singing on a streetcorner. The point is, again, that clarification is needed.
At base, "what if music is really super-important and powerful" is not a hypothetical question; it is a historical one. But it's not necessarily a physics question, and it certainly is not a matter of weaponizing sound. In order to dig into the issue more deeply, you have to consider just what you mean by "music" as opposed to "sound" or "noise".
3Hi and welcome to WB! I think this is a really interesting question but as it stands it's VERY broad. When you say influence reality are you talking about a "song of the world" type thing where musicians can cast spells/magic or are you after voice/music interface for technology? Any details you can give to narrow down the scope would be great. On a site like this being specific is key! :) – Liath – 2014-09-30T13:13:47.977
Cool question, but I agree with @Liath some more details and more specific ideas of what you are looking for would be great – bowlturner – 2014-09-30T13:21:06.233
I agree with Liath, it's a very good question, you just need to find specific instances of what you want. Ask us if you have questions. Look at the [faq] for help asking questions. – DonyorM – 2014-09-30T14:15:31.123
Did you consider to add a psychic component? That sound only has power when someones brain interprets it as music and the effect is rather a subconscious psychic power exerted by the listener? That would solve the problem of any kind of noise causing uncontrollable effects. – Philipp – 2014-09-30T17:33:09.720
Not quite what you're describing here, but you might find the Spellsong Cycle series interesting. Music (of any sort) has great power there, although sound, in general, doesn't.
– Bobson – 2014-10-01T12:26:09.757Very nice question; I find the lack of citations of lucasfilm' "loom" disturbing. – guido – 2014-11-10T16:06:35.077