The speaker is awfully interested in who gets to talk in this poem. In the first stanza, he says he wishes that he could express his thoughts, but he can't. Then he describes the kids who are shouting and playing, and the sailor who is singing. These guys get to express themselves – why can't our speaker? Then, in the third stanza, we start to figure out why he can't talk: it's because his friend's "voice is still." Our speaker is all choked up on grief that he doesn't know how to express.
- Lines 3-4: The speaker uses synecdoche, or substituting a part of something for the whole thing, when he wishes that his "tongue could utter." Because, yeah, your "tongue" is the part that does the speaking, but not on its own, unless you're talking in your sleep or something. The speaker uses metaphor when he says that his "thoughts" "arise," since thoughts don't literally move around.
- Lines 5-6: The "fisherman's boy" is "shout[ing]," but we don't know what he's saying – it may be utter nonsense as he's playing with his sister. The "O" that opens the line is just as empty of meaning as the shouts of the boy.
- Lines 7-8: The sailor's song is likewise without words – we don't know what he's singing; it might as well be "Tra-la-la." Again, the speaker's "O" that opens the line is just as meaningless. It's as though the speaker is starting to wonder if utterances – shouts, songs, or poems – have any meaning at all.
- Line 12: Ah, here's why the speaker is so down on utterances lately. It's because of his friend's "voice that is still." Again, he's using synecdoche by imagining that his friend's "voice" stands in for the whole guy.