As it was said in the comments and in the first answer, the sentences without who/which in your examples are wrong.
But, speaking in general, there is really a rule that comes to mind when looking at your examples. This rule regards to the relative (a.k.a adjective) dependent clauses. This rule states:
You can omit the relative pronoun of a relative clause, when this relative pronoun plays a role of an object in this relative clause.
For example, in the sentence
The ball which I bought this morning is quite good.
we can take the relative clause
which I bought this morning
and get rid of the relative pronoun which since it is an object of this relative clause.
The result will be:
The ball I bought this morning is quite good.
The problem with your examples is that relative pronouns in the relative clauses are not objects of these clauses. For example, in the sentence
A girl who had a sweet voice.
the word who is not an object, but a subject of the relative clause. The same applies to the other two examples. So we cannot omit the relative pronouns in these cases.