Question

let her sing..(change into passive voice)

Answer 1

the song must be sung by her...
hope helps

Answer 2

Let the song be sung by her.

     The above sentence when changed into passive voice becomes an imperative sentence which in general implies a request or command. Imperative sentences beginning with ‘let’ usually imply a request, suggestion, or desire.

     While changing an imperative sentence in active form to passive form, the following rule is applied:

    Let + object + be + past participle.

     In this sentence, the object is ‘song’, ‘be’ is the verb and ‘sung’ is the past participle of the verb sing.