You got to love a speaker that says eleven words in the first two lines before passing the poem off to a strange old man who likes to hang around outside wedding celebrations. But in all seriousness, the speaker of the poem is basically a narrator who sets up the action in this dramatic poem. The narrative has two levels: there's the story of the Mariner and the Wedding Guest, and then there's the story-within-a-story of the Mariner's voyage, which takes up most of the poem. We can comment briefly on the nature of the narrator of each story.
The narrator of the poem has a third-person limited perspective. That is, he knows the thoughts of only one of the two characters, the Wedding Guest. But he stays out of the way for most of the poem, and we don't learn the Wedding Guest's thoughts until the end, when we discover how he became a sadder and wiser man. At the beginning of the poem, the narrator uses one of the most ingeniously simple devices we've seen: "It is an ancient mariner…" "It is"! Brilliant. He doesn't even present the narrative as a story, he just points and says, "Look. There he is – that old sailor. Have fun, kids." Easiest narrator job ever. Then again, one of the more interesting questions in the poem is: is the narrator the "sadder and wiser" version of the Wedding Guest? Are they the same person? That, at least, would help explain how the narrator knows his thoughts at the end.
Both the narrator and the Mariner use old-school language, with words like "stoppest," and, our favorite, "eftsoons." And by old school, we don't mean nineteenth century, we mean closer to fourteenth century. Coleridge writes the poem in the voice of popular English balladeers, like the person who sung the famous ballad "Sir Patrick Spens." Many of the famous English ballads didn't have a single, named author. Instead, they were collective works that were revised and re-revised through the years, as different performers added their own touches.
As for the Mariner, he's also a true performer. In fact, his position mirrors that of the popular balladeer, in that he travels from place to place repeating his tale to different audiences. Except that he finds his audiences using his Troubled Soul Detector. Like any oral poet worth his salt, the Mariner uses lots and lots of repetition so that the listener will remember important things that happen. So, for example, the lines: "The ice was here, the ice was there, / The ice was all around." These lines might make a reader shout, "We get the point! Lots of ice!" but for the listener without a text, it helps him keep the elements of this very long story in place.