"The text introduces students to various regions of the United States and acquaints them with the history and culture of the sounds that make it unique." would have been clearer.
But anyway, an author who said "the different regions" would have been obliged to deal with all of the regions. Omission of the definite article admits the possibility that only some regions will be covered in the text.
It is hard to be certain about the scope of meaning of "sounds" without more context. But in view of the phrase "history and culture" the sounds that make regions unique would probably include characteristic music, for example blues in the south, bluegrass in Appalachia, jazz in New Orleans, and zydeco in Louisiana. Other cultural sounds that make a region unique might be characteristic regional idioms, slang and other regional usages such as "y'all" and "all y'all" in the south.
Accents might also be included, but probably not given the difficulty of describing accents to a general audience that is unfamiliar with linguistic technicalities such as the representation of phonemes.
Student or Learner