What passing-bells for these who die as cattle?
Only the monstrous anger of the guns.
Only the stuttering rifles' rapid rattle
Can patter out their hasty orisons.
No mockeries now for them; no prayers nor bells,
Nor any voice of mourning save the choirs,--
The shrill, demented choirs of wailing shells;
Orisons  are prayers. 
 The only “prayers” these dead and dying men receive are the the sound of the guns, which not only take life, but metaphorically recite the last rites. 
  The “rifles' rapid rattle” is  alliterative  with the repeated ‘r’s, and also  onomatopoeic . The words mimic the sounds of the guns, as they drown out the prayers for the fallen soldiers. 
 In contrast the description “hasty orisons” sounds soft, with sibilant, whispering “s” sounds.