In Wilfred Owen's "Anthem for Doomed Youth," the author refers to the tolling of bells.
It has been a common practice in England for hundreds of years to toll church bells at someone's passing. It occurs still today, in some places in the United States, and even at memorial services for those lost a sea. The tolling is a slow, mournful ringing. With this in mind, we can assume that Owen's reference to bells questions how the bells will be rung on the battlefield for all those who have died.
The answer for "who will serve as passing bells" is found in the first two lines of the poem—the guns blasting will have to serve also as the bells:
What passing-bells for these who die as cattle?
Only the monstrous anger of the guns.
"Doomed youth" cannot toll bells for themselves as they are dead. The "shires" are far away in England, and they cannot toll bells on the battlefield for those who have fallen—if a body is returned home, then it will happen. "Orisons" or prayers are quiet and would not serve to draw the necessary attention to the loss of lives. "Brows" is the "throw-away" answer. In context, the brows belong to the women left behind as the men go off to war, the same women who now will grieve.
Where the guns act as the tolling bells, the rifles rattle off prayers. It is as if in the midst of war, the aspects of society that are in place at one's death (the tolling of bells or the whispered "orisons" or prayers) are replaced by elements of the war, by necessity of where these men find themselves.
Only the stuttering rifles' rapid rattle
Can patter out their hasty orisons.
The best answer to this question is "guns."