“If you don’t keep quiet I shall punish you”,he said to her in a calm voice. The indirect sentence is

He warned her to punish if she didn’t keep quiet calmly

He said calmly that I would punish you don’t be‘quiet

He warned her calmly that he would punish her if she didn’t keep quiet

Calmly he warned her that be quiet or else he will have to punish her

He said to her whether he would punish her.