Our Adonais has drunk poison—oh! What deaf and viperous murderer could crown Life's early cup with such a draught of woe? The nameless worm would now itself disown: It felt, yet could escape, the magic tone Whose prelude held all envy, hate and wrong, But what was howling in one breast alone, Silent with expectation of the song, Whose master's hand is cold, whose silver lyre unstrung.
The speaker has moved from quiet mourning to anger. He says that our "hero," Adonais-Keats, drank poison.
Our speaker is using a metaphor here. The poison is criticism.
And he has some very choice words for the critic who wrote those words. He calls him "deaf and viperous murderers" (a viper is a snake) who used the poison of their words to create a "draught" (drink) of woe.
As if that wasn't enough, he calls the critic a "nameless worm" because he wouldn't put his name on the criticism. The speaker thinks that's pretty cowardly.
He continues his tirade: the speaker says that the critic felt the "magic tone" in Keats' writing, but criticized the poems anyways because he was envious, full of hate, and just plain wrong.
Those "magic" words lived inside one person (Keats); he says they were "howling in one breast alone." We guess that makes Keats irreplaceable.
Those words are still in there, he figures. They wait, silently, but their "master's hand is cold." He can no longer write them.
The speaker continues to use music as a metaphor for poetry. The speaker compares Keats' death to a musician whose "silver lyre" is "unstrung." The instrument is broken. Keats, the instrument of the poems, is also broken.