So many speakers to choose from, so few lines to contain them. Where most poems of this length would only find room for one point of view, this one crams in three.
The first is the speaker of lines 1-2 and the parenthesis of line 7. We don't know a lot about this person except he or she sets the stage and appears to hear both the dead man and the other people (even though the first thing the speaker says is "Nobody heard him"). There's maybe a hint that this speaker judges the dead man for "moaning," but otherwise he or she's a mystery we'll never solve.
The second speaker is the dead man himself, as we find out when he says, "I was much further out than you thought" (3). Somehow despite being dead he can't (or won't) shut up. What does this mean, really? Is he a ghost that other people can't hear? Is his corpse talking? Is he in the after-life commenting on his recent demise? All we can really know is that he's a dead man trying to set the record straight about his lonely struggle, only the other people (the third voice) aren't listening to a word.
The third speaker may actually be more than one person, as "they said" suggests. They chatter about the dead man's character and his final moments, apparently getting everything wrong. They could be friends or work acquaintances, but the poem doesn't really tell us their identities, and we're not sure it matters in the first place. They remain a general "they," those frustrating other people who never quite understand you. Ugh.
All these missed connections come together to show how you can be lonely in a crowd or dying in the middle of a bunch of people who would help you if they knew. It also shows how hard it is to see when someone needs help. Everyone in the poem seems to have their own needs and interests that keep them from really communicating with each other, which—it's undeniable—contributes to tragedy.