To answer the question, one first needs to understand what rhythm is.
Rhythm is the pattern, or flow, of sound created by the arrangement of stressed and unstressed syllables in poetry. A poet can also create rhythm through his/her choice of words used (repetition for emphasis), line breaks (pause the thought), and punctuation (again, pause of thought). The typical pause in the middle of a line of poetry is called a caesura. This effect can allow the reader a place to take a breath (as used in epic poems of the Anglo-Saxon period) or as a place of reflection.
Therefore, a poems rhythm is what dictates the speed of how a poem is read or spoken aloud.
In his poem "The Voice", Thomas Hardy is speaking about the lose of a woman and her love. Through his cleaver use of punctuation and recollections of the past, Hardy is able to slow the rhythm of the poem to provide the reader with a sense of longing and desire for the voice of a woman to call.
the rhyme scheme is ABAB, creating a cyclical structure --> rhythm is repetitive, cyclical, representing the way people can become locked into grief, inescapable process of emotions ect
in the final stanza, the anapaestic metre is broken down with use of caesura (punctuation within a line) : 'Thus I: faltering foward'. this disruption in rhythm implies Hardy is becoming physically worn by his grief, he's exhausted!
contrast this to the rhythm of the last stanza of 'The Going'. here, the last line of the first stanza is exclamatory:
'To gain one glimpse of you ever anon!'
as is the last line of the poem:
'Not even I—would undo me so!'
-> a cyclical strcture is created, sense of exasperation repeated at beginning and end implies Hardy can't escape the shock, frustration of Emma's death.
compare the two (exhaustion v exasperation) and you have evidence of Hardy's turmoil and confusion (typical display of grief)