Here's why:
* Subject: "Hills" is the noun acting as the subject of the sentence.
* Verb: "Are" is the verb, indicating the state of being.
* Predicate Adjective: "Alive" describes the state of the hills.
* Prepositional Phrase: "With sound" modifies the predicate adjective "alive."
Let me know if you'd like to explore the parts of speech within this phrase in more detail!