The use of these expressions is inappropriate when they are used to mean “somewhat” or “in a sense” but appropriate when they are used to mean “an approximation of” or “characterized as.”
Right: Music can be a kind of tranquilizer.
Wrong: Music can be kind of a tranquilizer.
As a rule, if the noun following “kind of” or “sort of” is abstract, consider the “kind of” or “sort of” phrase singular—and don’t preface it with an article.
Examples: What kind of movie is it? That kind of movie leads to aggressive behavior.