There's no room for laughter during war. The people have been left bitter by the fires and bombs of the conflict. Levertov uses some pretty stark imagery to remind the first speaker that war leaves destruction in its wake. And that's no laughing matter.
All the bones were charred. it is not remembered. (18-19)
After being asked if the people of Vietnam used bone and other fine materials for ornament, the second speaker again reminds the reader of the realities of warfare involving bombs. It leaves few remnants and destroys everything, from the ornaments of fine living to the people who used them.
When bombs smashed those mirrors (25)
This is the first mention of an actual implement of war: a bomb. It places the poem into context. It had to have occurred in an era when explosive weapons large enough to destroy a whole country were used. It modernizes a poem that has, up until now, relied on images that could take place in any century.
It is silent now. (31)
Much like line 16, line 31 reminds the reader that war leaves many casualties, all of whom will never speak again. That means they'll never be able to communicate traditions or pass down stories, which the speaker desires in order to better understand the people of Vietnam. War, then, can destroy an entire culture, leaving no way to remember it.