By describing their hearts as "light" the speaker is indicating that they were, once, innocent and happy. Now, they're stone: heavy and cold. What's more, they are no longer living. The contrast between "light" and "stone" reminds us that war destroys happiness and life. And that's not fair at all.
When peaceful clouds were reflected in the paddies and the water buffalo stepped surely along terraces, (22-23)
This peaceful scene is an echo of the past, says the speaker. It's a pretty idyllic image and, by using it, Levertov reminds the reader that we lose peace when we choose war. And, by doing so, we do a great injustice to nature itself. Now, there are no more reflections of nature to be scene in Vietnam, because it has all been destroyed.
there was time only to scream (26)
The people of Vietnam didn't have time to do anything but scream when the bombs started falling. The first speaker asked about the song and speech of the people, but those songs were replaced by these screams. The people were unaware that they were soon to lose their lives, and didn't have a chance to protect themselves or their traditions. Now, the poet speaks for them, in hopes of preventing this injustice from occurring again.