Mable walked closer to the figure after calling to him, but paused once she saw his - her? - face.  That, in addition to the deep voice, surprised her.  This person looked like a woman from the front, but sounded like a man!  Was it appropriate to ask?

"You whistle very well," she said, continuing on closer to see this person better.  "It was so loud!  Do you think you could teach me how to do that?  My friend knows how to whistle, but he won't teach me!"