(1) euphonically, that is, understanding it based on its good-sounding-ness
(2) formally, that is, according to its set-up.
A good symphony and a good mass can be understood on both levels. In fact, it's really not understood until it's understood on both levels. And with especially good symphonies and masses, it will be difficult to understand it as (1) without understanding it as (2). A really good composer is not obscure about the formal beauty of his piece. He draws you into its formal beauty, luring you in with its pleasant sound.
Take Beethoven. His music is engaging at any given moment, but it's difficult not to understand the overall drama of the sonata form present in the work. Beethoven is a Herodotus or a J. K. Rowling. The moment you're listening to is exciting, but he draws you into the bigger picture, the garden, wilderness, and garden-city of a symphonic first movement. Beethoven, good story teller that he is, lengthens your attention span.
Think of it this way—for a three-year-old, as he listens to Lord of the Rings being read to him, chances are he doesn't understand the big picture: the quest to destroy the Ring, Sauron trying to thwart Gondor, Sauraman, Rohan. But that doesn't mean that he can't feel the excitement during Helm's Deep. It doesn't stop him from looking like a stone when Frodo's finger's getting ripped off with teeth.
A twelve-year-old, on the other hand, is likely to understand the big picture. It isn't simply an instantaneous enjoyment of the moment that keeps him going. It's the unresolved tension way back in Book One that holds an inexorable grasp on his attention while he's reading Book Five. If you reflect for a moment, you'll realize it's really quite amazing that something he might have read three months ago is informing how and why he reads right now.
But why does the knowledge that The Ring Must Be Destroyed really matter that much? Couldn't it be that three-fourths of the way through the book, Frodo slips and falls, gashes his head, drowns in a river, and the ring gets forgotten for another thousand years? No. This couldn't happen any more than Sauron dying at the end from food poisoning. It's because the twelve-year-old recognizes form that he knows that none of these things would ever happen. That's why we feel gypped when an author pulls a deus ex machina. That's why everyone hated the last episode of Lost.
People recognize the form of stories. They know certain things are in the realm of possibility, and certain things aren't. Within that realm, what the author does is exciting. The bad guys might just win - that's in the realm - but the piece might also end on a picardy third, so to speak.
Which brings me back to my original point. In most cases (and, I grant, not all), the reason someone gets bored with a symphony is not because the symphony is boring, but because he is a boring person. He does not recognize a form. He's like someone who doesn't appreciate an exciting story when he hears one. But we must have pity on this sort of person and try to educate him to enjoy the tension of a story.
That's why Beethoven lengthens your attention span. If you recognize the form, if you understand what's in the realm of possibility and what isn't, you'll know that the symphony has its natural crescendo, climax, and resolution, just like a story. And if you realize that, you're the twelve-year-old of music, now, no longer the two-year-old. You don't simply have an instantaneous enjoyment of the music. What happened in the first few bars is informing how you see the end of the movement.
Why is that so hard? What I read a month ago at the beginning of War and Peace sticks in my head word for word as I read about characters at the end of the novel. But I have a hard time identifying a simple tune that shows up at the beginning of a piece six minutes later. Why is that?