I believe you are asking about the very first time that Winnie's grandmother and the man in the yellow suit interact with each other. That happens in chapter four. The question that he asks her is as follows:  

"Wait!" said the man at the gate. He had stiffened, and his voice was eager. "You've heard that music before, you say?"

Winnie, her grandmother, and the man in the yellow suit are all standing in the front yard of the Foster home. It's getting close to evening, and Winnie is out watching and trying to catch fireflies. The man in the yellow suit approaches Winnie and asks her how well she knows the people of Treegap. During the conversation, Winnie's grandmother comes out and tells the man that the Foster family has been in Treegap for a long time but they don't know everybody. At that moment, the group hears the tinkling of some music coming from the woods. Winnie's grandmother thinks it is elf music, but Winnie thinks that it sounds like a music box:  

"It sounds like a music box," said Winnie when it was over. "Nonsense. It's elves!" crowed her grandmother excitedly.

It turns out that Winnie is correct, but the fact that her grandmother has heard it before spikes the interest of the man in the yellow suit. He asks for clarification from the grandmother about her recollection of the music in the woods.