My answer:
I have heard her sing this song several times.
-> She has been heard to sing this song several times by me.
People saw him steal your car.
-> He was seen to steal your car.
The teacher is watching them work.
-> They are being watched to work by the teacher.
He won't let you do that silly thing again.
-> You won't be let to do that silly thing again.
Peole don;t make the children work hard.
-> The children aren't made to work hard.
They made him work all daay.
-> He was made to work all day.
The detective saw the wonman putting the jewerlry in her bag.
-> The woman was seen putting the jewelry in her bag by the detective.
The errorists made the hostages lie down.
-> The hostages were made to lie down by the errorists.
Police advise drivers to use an alternative route.
-> Drivers are advised to use an alternative route by the police.
She helps me do all these dificult exercises.
-> I am helped to do all these difficult exercises by her.