Into the solemn church, and scatter the congregation, Into the school where the scholar is studying, (3-4)
In a church, people generally follow certain rituals and ceremonies. So, when the congregation is scattered there's a disruption of a normal order. Likewise, a scholar at his studies makes us think of orderliness and discipline (or at least we'd hope so, studious Shmoopers!). This military band – with its own kind of discipline – comes by and disrupts that, too. All these disruptions show us the power of the war to disrupt the normal order of society.
Over the traffic of cities—over the rumble of wheels in the streets; (9)
This overhead view of the city, with its traffic and its rumbling, gives us a broader view of the daily routes and routines of city people. But our speaker tells us that the sound of drums and bugles seems to cover the whole city. The music is now the dominant sound: it's definitely not blending into the background.
No bargainers' bargains by day—no brokers or speculators—would they continue? Would the talkers be talking? would the singer attempt to sing? Would the lawyer rise in the court to state his case before the judge? (11-13)
More disruption! Here we get a list of people defined by what they do who are being kept from doing that thing. It's in the normal order for a bargainer to bargain, a talker to talk, a singer to sing. But that band just won't leave things be.
Make the very trestles shake under the dead, where they lie in their shrouds awaiting the hearses, (20)
Yikes – even the rituals of death are disturbed by the music. Death is normally an event that we respond to with very set, orderly procedures. Even within this very line, we read about the shrouds that cover the bodies and the hearses that take them away. But it turns out that even the sacred rituals of death are threatened or shaken up by war.