Generally speaking, when looking at a poem for figures of speech, I usually steer students toward finding metaphors and similes. That's a bit tougher to do with "The Listeners" because the poem doesn't make extensive use of metaphors. There is a metaphor about halfway through the poem, though.  

Hearkening in an air stirred and shaken   

In the above line, air is being compared to something that could be stirred and shaken. Air can be diffused and blown, but it is not something that can be picked up in order to shake.  

The poem is very concerned about mood, and the author uses alliterationto help convey the mood. When I think about the sound of silence, I think about the "s" sound. Perhaps it is because of that letter in those two words. Perhaps it is because that's what wind sounds like to me, or perhaps it is because that's the sound a person makes when "shushing" another person. "The Listeners" makes a point of telling readers that silence continually greets the visitor. The alliteration of the "s" sound throughout the poem highlights and accompanies that silence.  

And how the silence surged softly backward, 

The "s" alliteration is even present in the first line that I quoted too. Line three is also a very alliterative line.  

Of the forest’s ferny floor: 

A final figure of speech that I like in this poem is anaphora. Anaphora is a deliberate repetition of the first part of the sentence in order to achieve an artistic effect. In "The Listeners," I like the anaphora because it relates to the alliteration I spoke of earlier.  

Stood listening in the quiet of the moonlight   
   To that voice from the world of men: 
Stood thronging the faint moonbeams on the dark stair