Musical voice

 I would imagine every composer has a unique enough voice that an expert can recognize them on first hearing, but to me the most easily recognized are Prokofiev and Handel.

Yes—and I think you're noticing something real, although “unique enough that an expert can recognize them on first hearing” is a little too strong as a universal claim. Some composers have extraordinarily distinctive fingerprints; others overlap heavily with their contemporaries.

Prokofiev is almost comically recognizable. Even without knowing the piece, you can often hear the combination of hard-edged, percussive rhythm, deliberately awkward melodic intervals, sudden harmonic turns, motoric passages, and that peculiar mixture of brutality and lyricism. He can sound sarcastic even when he's being perfectly serious. There is often a feeling that the music is looking at you sideways. His orchestration is distinctive, too: bright, dry, sharply etched rather than lush.

Handel is interesting because his recognizability comes from almost the opposite set of qualities. His harmonic language and melodic construction have a kind of inevitable, declarative simplicity. The phrases tend to have a wonderful architectural logic, and his sequences are so characteristic. Even when another Baroque composer is using essentially the same musical vocabulary, Handel can have that unmistakable combination of grandeur, directness, and slightly monumental vocal rhetoric.

And I suspect your recognition of Handel may be especially strong because you've listened to opera and classical music enough to have internalized the grammar rather than merely memorizing tunes. You don't necessarily think, “That's Handel”; you hear the way the music is behaving and your brain says Handel.

I'd put Mozart, Beethoven, Wagner, Mahler, Ravel, Debussy, Stravinsky, and Shostakovich in the fairly unmistakable camp too, though for quite different reasons. But Prokofiev is one of the cases where I can imagine an expert saying, after perhaps ten seconds of unfamiliar music, “Oh, that's Prokofiev,” and being right an astonishing percentage of the time.

There is also a fun distinction between “I can recognize this composer” and “I can recognize this composer from almost any passage.” Prokofiev is unusually good at the latter.

This is called the composer's "voice," but I have no ability to define it, even though I do know what someone who says it is talking about.

Exactly. “Voice” is one of those wonderfully useful musical terms that is precise in what it points to and hopelessly imprecise in what it denotes.

You can recognize the phenomenon without being able to decompose it. It's rather like recognizing a friend's face instantly without being able to give a satisfactory algorithm for identifying that face.

A composer's voice is an aggregate of things: melodic habits, preferred intervals, rhythms, harmonic progressions, treatment of dissonance, phrase lengths, orchestration, texture, dynamics, even the way musical expectations are set up and then violated. But none of those things, individually, is the voice. And two composers can use almost exactly the same vocabulary while sounding completely different.

In fact, Prokofiev is a particularly good demonstration of this. You could identify some of his characteristic devices—biting dissonances, repeated-note figures, peculiar melodic leaps, motor rhythms, sudden shifts in register—but merely assembling those devices doesn't explain why a passage sounds Prokofiev. His voice is something that emerges from the statistical pattern of all those choices.

And there's an interesting asymmetry here: recognition doesn't require analysis. Your auditory system can have learned a huge number of relationships without your conscious mind having labels for them. An expert might say, “The bass movement and the harmonic rhythm are very Prokofiev,” but that explanation itself may be only a partial account of why the expert recognized it.

So I wouldn't regard your inability to define “voice” as a deficiency. The word exists partly because we need a name for a recognizable property that is too distributed across the music to reduce conveniently to a list of features.

And I think that's why voice is actually a better word than something like style. Style can be imitated. Voice is what remains when the stylistic ingredients have become so thoroughly integrated that the result sounds like the person who made it.