The voice had the sparkle of a mountain stream who thought so.

The voice had the sparkle of a mountain stream who thought So special. 

They eyes have it

Answer:–

The narrow thought so.

The voice referred to here is that of the girl who was traveling with the narrator in the same compartment.

The blind narrator on his journey met a girl whom he started to like from the very beginning.

Since the short train journey did not give him enough time to draw a mental image of the girl, he simply used his ears.
The narrow considered the voice of the girl to be special as her voice reminded him of the sparkle of a mountain stream.
He grew so fond of her voice that he was ready to sit with her for any length of time, just to listen to her talking.