Epics are long, narrative poems, often referencing events in history or mythology. If the people had their own, that would signal a long tradition of art, a tradition of preserving culture. If the first speaker were able to read their epic poems, he would gain a lot of understanding about the history of the people.
Did they distinguish between speech and singing? (9)
Here, the first speaker ends his or her round of questioning with a query into the way the people of Vietnam communicated. This seems to indicate that perhaps the people of Vietnam used song as a way of speech, which would make them seem somehow even more peaceful. At least, that's the way the speaker is imagining it.
maybe fathers told their sons old tales. (24)
After being asked about an epic poem, the second speaker responds that it's possible all they had were stories, passed from family to family. This is another way of preserving the culture and history of a people; it's not always considered art, but it may have worked for them. Still, the second speaker can't be sure. Whether it was formal poetry or family lore, all stories have been lost in the war.
It was reported their singing resembled the flight of moths in moonlight. (28-29)
Here is the only mention in the entire poem of a lasting piece of their culture. While in actuality, Vietnamese culture and art haven't been wiped out, in the poem they have. So this mention of singing is really all the speakers have to go by. What "moths in moonlight" sound like is up for debate, but we can be sure it is a gentle, delicate sound.