Strong alliteration helps to give this poem its rich music. Even before alliteration comes in, this poem's rhymed triplets and urgent trochaic meter feel intense and brightly colored. Now listen to what happens when Browning gets alliterative within that already striking structure:
Did young people take their pleasure when the sea was warm in May?
Balls and masks begun at midnight, burning ever to midday,
When they made up fresh adventures for the morrow, do you say?
After those first punchy /p/ sounds, the weaving /b/ and /m/ sounds make these lines sound as hypnotic and non-stop as the Venetian parties they describe.
Browning's alliteration can be quiet and sinister, too:
What? Those lesser thirds so plaintive, sixths diminished, sigh on sigh,
Told them something? Those suspensions, those solutions—"Must we die?"
Those commiserating sevenths—"Life might last! we can but try!"
The sibilant /s/ sounds here feel close to creepy. The "voice" of Galuppi's toccata seems to whisper as it warns the partiers of their inevitable deaths; the delicate /l/ alliteration of their response suggests their impossible longing not to.
When the speaker imagines Galuppi addressing him in turn, the maestro's voice gets a little less subtle:
"Dust and ashes, dead and done with, Venice spent what Venice earned.
"The soul, doubtless, is immortal—where a soul can be discerned.
Those thumping /d/ sounds feel like nails hammered into Venice's coffin.
Lively, dramatic sounds like these are a hallmark of Browning's poetry (and one of the reasons some of his more fainthearted contemporaries didn't know what to do with him). The alliteration here makes this poem as Browning-y as Galuppi's toccata is Galupp-y.