What softer voice is hush'd over the dead? Athwart what brow is that dark mantle thrown? What form leans sadly o'er the white death-bed, In mockery of monumental stone, The heavy heart heaving without a moan? If it be He, who, gentlest of the wise, Taught, sooth'd, lov'd, honour'd the departed one, Let me not vex, with inharmonious sighs, The silence of that heart's accepted sacrifice.
Someone else has arrived, with a soft voice and a heavy look "athwart" (across) his "brow" (forehead). He is mourning, too.
Some folks think this is Leigh Hunt, who was good friends with both Keats and Shelley.
Whoever he is, he leans over the bed, which the speaker calls a "mockery of monumental stone" meaning a mockery of a statue. With Keats dead on the white bed, the scene is as still as a statue. Perhaps Shelley wishes Keats was made into a statue.
The speaker calls Hunt "gentlest of the wise" and praises him for loving, soothing, and honoring Keats. If the mourner is Hunt, then the speaker won't "vex" (disturb) with "inharmonious sighs" (odd noises, referring to the speaker's own words) Hunt's silence at the deathbed. He knows that Hunt loved Keats and wants to leave Hunt in peace to mourn.
This perhaps refers to Hunt's silence in real life. He may have been to overcome by grief to do something more public, like write a 55-stanza poem, the way Shelley did. This stanza could be Shelley's way of saying that he is okay with Hunt's silence on the matter.