Jane
I hate to disappoint you, but I spend FAR more than 2 hours a week singing and don't lose any weight. I have two good friends who are both professional singers and who are about twice my size.
Perhaps if you are in a very active role - dancing etc. - then some of the two hours could be counted as aerobic exercise, but I doubt if you are fully active for the whole 2 hours every week.
What is the opera you're in? Where/when is it on?
Fiona