We know that Intransitive verb can't be used in passive voice. But I am sure that they can be changed into passive voice if there is no change of meaning. Here are some examples-

1.Birds fly.

=By birds are flown.

2.She sings.

=By her is sung.

2.He goes to school.

=By him is gone to school

or, To school is gone by him.

But I am not sure whether above passive sentences are correct or not. Now, I need your help. Please, explain the matter with argumens.
Your sentences are definitely not correct. You have said
AnonymousWe know that Intransitive verb can't be used in passive voice
but you seem determined to prove otherwise. Please don't try -- it just doesn't work. "By birds are flown" does not mean "Birds fly." In fact, it doesn't mean anything intelligible.

I'm not sure if this is the best explanation, but I would say that your sentences are incorrect because they have no subjects.

Compare "she sings" (intransitive) with "she sings a song" (transitive).
"She sings a song" can be expressed using passive voice as "a song is sung by her." (It's not eloquent, but it's grammatically acceptable.) "A song" answers the question "what is sung by her?"

"She sings" cannot be made into passive voice because nothing can be named that is being sung by her. "By her is sung" does not answer the question "what is sung by her?"

Does that help?

it is sung by him