Let us remember that a metaphor is a form of figurative language in which one object is normally compared to another directly, without the use of the words "like" or "as," as in a simile. Normally these two objects are very different, and the metaphor helps us to see one point of comparison. Thinking about this, this helps us to understand how this metaphor functions in this poem. Let us remind ourselves of what is said:No mockeries now for them; no prayers nor bells; Nor any voice of mourning save the choirs, –The shrill, dementedchoirs of wailing shells; And bugles calling for them from sad shires.
We can see that this metaphor is part of an angry line that refers to the dead soldiers who have "died like cattle" and are now only remembered by the "choirs" of "wailing shells." Because of their ignomonious deaths on the battlefield, they do not have a proper service. Instead, Owen imagines the sound of the "wailing shells" to be the only choir that these dead soldiers will ever hear. Thus the answer is "shells."