"A Birthday" is singing a happy song. It's a praise poem, really, one that celebrates the speaker's relationship with the divine. As such, we would expect it to make happy sounds—like birds chirping or kittens snoring—and we're not disappointed.
The first sonic clue to the speaker's buoyant mood comes in the form of alliteration: "Whose boughs are bent with thickset fruit" (4). Those B words ("boughs" and "bent") add a kind of rolling energy to the line and send us bouncing and bounding merrily along as we get more details about the speaker's celebratory mood.
More than alliteration, though, Rossetti relies on consonance to deliver the bulk of the poem's sound effects. Specifically, the S sound is super-special to the speaker's scintillating story. We hear S's in lines 6 ("That paddles in a halcyon sea"), 10 ("Raise me a dais of silk and down;") and 14 ("In leaves and silver fleurs-de-lys"). The effect is a kind of peaceful whisper that runs in the background the poem like a kind of peaceful sound effect.
As such, the summary of these several S's soothes our senses with some sweet sounds. For a speaker celebrating peace and connection in her life, we'd say that they're some super-sensible sounds to select.