The speaker of this poem is either playing or listening to a toccata (a piano exercise meant to display the player's virtuosity) by Galuppi, an 18th-century Venetian composer. The piece is spoken directly to the composer.
He begins with a display of melancholy - "this is very sad to find!" - at what he has discovered in Galuppi's music. It makes the speaker, who admits he has never left England, imagine life in Galuppi's Venice. He paints for himself its sea-defined geography and thinks of how lovers must have enjoyed each other in such a landscape. He then imagines how the lovers would talk while Galuppi played a toccata for them.
He believes that Galuppi's music at first would drive the lovers towards the melancholic question, "Must we die?" However, when the music changed, the lovers were given sudden hope to persist, and the speaker then imagines a vacuous conversation in which the lovers celebrate their life and love together. The lovers must have praised Galuppi for giving them such hope, he imagines.
His mind then goes to the truth, which is that "death stepped tacitly" in and took all of Venice away. As the speaker considers this idea, Galuppi's music suddenly seems "cold" to him and frightens him. The music continues to stoke his own hopes that he'll "not die, it cannot be," but also reminds him that Venetians who felt the same way died nevertheless. In the end, what he hears from Galuppi's music is "dust and ashes," which makes him "feel chilly and grown old."
At the center of this poem are questions about art, particularly the value of enjoyable, escapist art. To best understand the theme, it is useful to first understand the description of Galuppi's work as provided by this speaker. When he imagines the Venetian couple listening to Galuppi's music, he first sees them growing melancholy as he plays his "suspensions" (notes or musical phrases that create tension because of a lack of resolution). Such melancholy leads them to confront their mortality. However, what Galuppi then offers is an escape in his "commiserating sevenths" (a satisfying musical interval), which let the couple then think themselves immortal. Because Galuppi offers music that resolves the tension of death for a while, the couple loves and praises him.
However, the truth cannot be avoided: death will come. The speaker in his imagination goes through the same process as that couple, though in the opposite direction. He begins the poem by imagining the glorious history of Venice, but ends the poem accepting that death comes to it. Time cannot be stopped and greatness falls. The irony of Galuppi's music is that, by providing a pleasant escape to people of his time, it now brings the contemporary narrator sadness. It is worse to hear the pleasant resolutions of his chords and recognize that such pleasant distraction was a lie, than it would have been to hear unresolved, melancholy music. What concerns him is that Galuppi's resolved chords and escapist sensibility proved unable to stop the incessant march of time.
In this way, Browning makes a strong statement about the limits of art: where it might bring solace and comfort in the moment, it is limited in its ultimate effect. It cannot save lives or change what is to come. Galuppi has the power as an artist to help these people confront their mortality - the first stage of his music teaches just that - but chooses to resolve that tension into a false reaffirming of life. The question becomes, then: what is better for an artist, to offer an escapist fantasy or a serious reminder of mortality? Characteristically, Browning does not offer a definitive answer, though he does note that even the former gains a sad, creepy air in hindsight when the irony of its ineffectiveness is made apparent.
One should not see this narrator as representing Browning. Not only has the narrator never been out of England (whereas Browning, who published this poem in 1855, had lived in Italy with Elizabeth Barrett Browning for several years), but the speaker is, according to the seventh stanza, a man of science and mathematics. It is a useful choice, since putting such contemplation in the mind of an artist would almost necessitate a more explicit consideration of the purpose of art, whereas this man, not an artist by trade, merely wishes to explore the effect of music that pleases him. He obviously enjoys Galuppi's work and gives the composer great credit for his spontaneous toccatas. But because he has the hindsight to know that Venice fell despite the composer's work, he is forced to confront the limitations of that which pleases him. Just as the pleasure of our bodies will mean little when we are "dust and ashes," so do the pleasant chords lose their meaning, and so the music becomes "cold."
One final element that reinforces the poem's themes and also shows Browning's implicit presence is the form. The poem is composed of 15 rhyming triplets, an extremely rare and difficult form. Further, the lines are composed of octometer, which requires a certain virtuosity of language. In a way, Browning is composing his own "toccata," a piece designed to show virtuosity rather than long-form composition. The form also works to parallel the speaker's final recognition that he has "grown old." Even in the prettiest and most impressive poetry lies a reminder of our mortality. Galuppi's music ultimately reveals, generations later, that man is doomed to die; the speaker, in the midst of his own showing off, has remembered that same truth.