Here is one of my post/blogs, which exists on another site:
Instrumental music is not, literally, about any particular "narrative." This doesn't mean it has no other meaning, such as evoking strong "emotional states," as in Schoenberg's Transfigured Night, Five Pieces and Mahler's symphonies.
Instrumental music, "musical sound", when divorced from "literal action" and drama, lost its connection to explicit meaning, and was revealed for what it is: a non-representational medium, the abstract evocation of "inner" states of being, which, coincidentally, is exactly what "abstract art" does: it reveals the artist's, and by empathy, the viewer's inner emotional state of being.
Music gradually divorced itself from drama over several centuries. Look at the rise of instrumental forms: the symphony, the concerto, tone poems, etc.
In instrumental Romanticism, although it was music divorced from drama, still had residual traces of drama, expressed as "dramatic gestures."
This "splitting" of drama from music opened-up a new can of worms, giving us the whole range of the non-specific "feelings" evoked by music, which are by their very "non-narrative nature" fleeting, transitory, and ephemeral, unclear, evocative, vague, and indefinable (meaning non-narrative).
Still, this is not a requirement for music to be expressive of emotion or states of being. To take matters even further into the fog, when we get into more modern music, I think "emotion" as a descriptive term begins to fail us. For example, in Schoenberg's Five Pieces for Orchestra, the "emotional gestures" expressed are so complex that we begin to experience them as "states of being," like anxiety, foreboding, fear, tension, awe, etc., creating in our minds, empathetically, a reflection of our own, and the artist's, "inner state of being."
Concerning modernism, it's true that in many instances the "evoking" of dramatic emotion, and dramatic gesture is absent (but certainly not always). Stockhausen evokes, for me, a sort of "Platonic classicism" in his Klavierstücke; with modernism, we must put aside our need for drama and overt emotion, and listen on the level of "pure abstraction," an enjoyment of color, sound, and timbre itself. In this sense, modern music is not "modern" at all; music has always been "abstract expressionism" when divorced from drama and opera.
So, in a sense, this is an "internal narrative" we share with the composer, but indefinable in literal narrative terms, because these are transitory, fleeting states by nature; simply "gestures of meanings."
Our general knowledge, and the historical context of a work can provide a source of "general narrative content" which can add greatly to the meaning of a piece, if only in our own minds. This always happens for me with Shostakovich (images of Soviet Russia) and with Webern's Op. 6 (Six Pieces for Orchestra), which always evokes in me grey images of Europe immediately preceding the World Wars. With Mahler, the Sixth Symphony snare-drum always evokes images of some malevolent military presence marching through our once-peaceful existence.
I think in many cases, the composer actually is composing with a specific narrative in mind, from his own emotionally-charged experience of events in his life, and then leaving it up to us to interpret it as we will; but we will never know for sure. That's the beauty of poetry; it is open-ended in meaning.
That's a useful distinction, I think; instrumental non-narrative music (containing "dramatic gesture") is more like poetry, whereas the explicit meaning and narrative of opera is like a story or novel.
Perhaps that's the reason opera seems to lend itself to an audience more easily; the "poetry" of instrumental music is an "inner" experience, more solitary in nature, like reading a book of poems by yourself. Maybe sitting there in the concert hall listening to instrumental music gave audiences too much idle time to think.