Whoever thought this up for a test question should have someone stand on his foot until he promises to never do it again. That being said, I'll try to answer your question.
In sentences similar to the one you cited, the words once and when are interchangable without significantly changing the meaning of the sentence. You do, however, subtly change the meaning. To my ear, the use of the word once implies a response to a challenge, or a change from what is expected.
"She was beautiful and well-dressed, and when she started to sing, the audience was enthralled."
"She looked like a homeless person who wandered in from the street, but once she started to sing, the audience was enthralled."
This is not a rule, just my opinion.