In "Echo," the speaker addresses their deceased lover. Apostrophe (the device in which a speaker addresses someone who can't respond) creates an intimate tone, as though the reader were eavesdropping on a private conversation. This "conversation" is one-sided, though: the speaker begs their beloved to visit them in dreams, but of course receives no response.
The title, "Echo," might also subtly allude to the ancient Greek myth of Echo and Narcissus. Echo fell in love with Narcissus but couldn't speak to him, due to a curse that made her capable only of repeating others' words. This allusion helps reinforce the idea that the speaker's lover can't hear them. The title also suggests that whatever experiences this couple shares in "dream[s]" are mere echoes of a former love; they aren't real in themselves.
Given the title, it's no surprise that "Echo" is full of repetition from the first lines onward:
Come to me in the silence of the night;
Come in the speaking silence of a dream;
These lines contain both anaphora ("Come [...] Come") and grammatical parallelism. They also repeat the word "silence." The repetitions create an insistent tone, conveying the speaker's desperate desire to see their beloved again. The emphasis on "silence" also helps establish a hushed, eerie atmosphere. Yet the oxymoron "speaking silence" implies that the "dream[s]," while soundless, communicate intense emotion. They're the speaker's only opportunity to see their lover again, so they're a welcome respite from the day-to-day reality of grief.
Repetition adds to the musicality of these opening lines, as does alliteration ("silence," "speaking silence"). These sibilant /s/ sounds give the language a whispering, intimate quality. One can imagine the speaker waiting till the dead of night to say these words aloud. Muted /m/ and /n/ consonance ("Come," "silence," "night," "dream") also contributes to the hushed tone.
The poem creates music in other ways, too. Rossetti uses the stately, familiar meter known as iambic pentameter (five-beat lines that follow an unstressed-stressed rhythm: da-DUM, da-DUM). However, she occasionally varies the meter for emphasis. For example, she begins these first two lines with trochees (stressed-unstressed feet) rather than iambs (unstressed-stressed), accentuating the word "Come" and making the speaker's pleas sound more forceful:
Come to | me in | the si- | lence of | the night;
Come in | the spea- | king si- | lence of | a dream;
The poem also follows an ABABCC rhyme scheme, which adds to its lyricism and emotional force. Together, these various sound effects create euphony: the poem is richly melodious and pleasing to read aloud.