The bugles in Wilfred Owen's "Anthem for Doomed Youth" come "from sad shires." Shire is the British equivalent of county. They are unlikely, then, to be playing anything directly related to battle.
The entire poem, from choirs to palls to funeral bells, is an extended metaphor of a funeral. The victims of WWI referred to in the poem will not be receiving traditional funerals at home. For instance, the sound of exploding artillery shells takes the place of church choirs traditionally heard at funerals.
Women will stare out of their windows looking for their returning men coming home, but the men never will. At the close of the day, the women will reluctantly draw their window blinds.
The bugles then, presumably, will play taps, if the choices you ask about are the only choices. They will play something associated with death.