Here are the sentences rewritten in the passive voice:

1. By midnight, all the escaped prisoners had been recaptured.
2. Has a date for the field trip been fixed by the school board?
3. Comments had been left under the post by lots of people.
4. The cause of the fire is still unknown to us.
5. These letters have been delivered to the manager by someone.
6. Homework has not been assigned by the teacher yet.
7. A huge sum of money has been taken away by the robbers.
8. The work had not been completed when we returned.
9. Have the flowers been watered by anyone?
10. How many books had been written by him before he died?