"The girl is being heard by the entire population."
Translation:Pigen bliver hørt af hele befolkningen.
I can't say whether, "....den hele befolkning" is correct or not, but to me it sounds unnatural in this particular sentence. I can not recall having heard similar sentences spoken in the word order that you propose. One example which I have heard in the street is, ("Two grilled hotdogs with all the trimmings, please".) "To ristede med hele svineriet, tak", and never "To ristede med det hele svineri, tak." It would be interesting to have a Dane comment on this.
What is it that is special about the adjective "hele" that allows you to use it before the definite form of a noun? For example, one would also say "hele bogen" or "hele dagen". But other adjectives would require splitting the article, as in "den dejlige bog", "den store dag", etc. Is "hele" alone in this category?